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and in the last appear as mere appendages to the end of the long and broad diapophyses: but the hamapophyses by no means disappear after the solution of their union with their pleurapophyses; they are essentially independent elements of the segment, and they are continued, therefore, in pairs along the ventral surface of the abdomen of the crocodilia, as far as their modified homotypes the pubic bones. They are more or less ossified, and are generally divided into two or three pieces.

The lumbar vertebræ are those in which the diapophyses cease to support moveable pleurapophyses, although they are elongated by the coalesced rudiments of such which are distinct in the young crocodilia. The length and persistent individuality of more or fewer of these rudimental ribs determines the number of the dorsal and lumbar vertebræ respectively, and exemplifies the purely artificial character of the distinction. The number of vertebrae or segments between the skull and the sacrum, in all the crocodilia I have yet examined, is twenty-four. In the skeleton of a gavial I have seen thirteen dorsal and two lumbar; in that of a crocodilus cataphractus twelve dorsal and three lumbar; in those of a crocodilus acutus and alligator lucius, eleven dorsal and four lumbar, and this is the most common number; but in the skeleton of the crocodile, probably the species called croc. biporcatus, described by Cuvier, he gives five as the number of the lumbar vertebræ. But these varieties in the development or coalescence of the stunted pleurapophysis are of little essential moment; and only serve to show the artificial character of the "dorsal" and "lumbar" vertebræ. The coalescence of the rib with the diapophysis obliterates of course the character of the "costal articular surfaces," which we have seen to be common to both dorsal and cervical vertebræ. The lumbar zygapophyses have their articular surfaces almost horizontal, and the diapophyses, if not longer, have their antero-posterior extent somewhat increased; they are much depressed, or flattened horizontally.

The sacral vertebræ are very distinctly marked by the flatness of the coadapted ends of their centrums; there are never more than two such vertebræ in the crocodilia recent or extinct in the first the anterior surface of the centrum is concave; in the second it is the posterior surface; the zygapophyses are not obliterated in either of these sacral vertebræ, so that the aspects of their articular surface-upwards in the anterior pair, downwards in the posterior pair-determines at once the corresponding extremity of a detached sacral vertebra. The thick and strong transverse processes form another characteristic of these vertebræ; for a long period the suture near their base remains to show how large a proportion is formed by the pleurapophysis. This element articulates more with the centrum than with the diapophysis developed from the neural arch; it terminates by a rough, truncate, expanded extremity, which almost or quite joins that of the similarly but more expanded rib of the other sacral vertebre. Against these extremities is applied a supplementary costal piece, serially homologous with the appendage to the proper pleurapophysis in the dorsal vertebræ, but here interposing itself between the pleurapophyses and hæmapophyses of both sacral vertebrae, not of one only. This intermediate pleurapophysial appendage is called the "ilium;" it is short, thick, very broad, and subtriangular, the lower truncated apex forming with the connected extremities of the hæmapophysis an articular cavity for the diverging appendage, called the "hind leg." The hamapophysis of the anterior sacral vertebra is called "pubis," 64; it is moderately long and slender, but expanded and flattened at its lower extremity, which is directed forwards towards that of its fellow, and joined to it through the intermedium of a broad, cartilaginous, hæmal spine, completing the hæmal canal. The posterior hæmapophysis, 63, is broader, subdepressed, and subtriangular, expanding

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as it approaches its fellow to complete the second hæmal arch; it is termed "ischium." The great development of all the elements of these hæmal arches, and the peculiar and distinctive forms of those that have thereby acquired, from the earliest dawn of anatomical science, special names, relates physiologically to the functions of the diverging appendage which is developed into a potent locomotive member. This limb appertains properly, as the proportion contributed by the ischium to the articular socket and the greater breadth of the pleurapophysis show, to the second sacral vertebra; to which the ilium chiefly belongs.

The first caudal vertebra, which presents a ball for articulating with a cup on the back part of the last sacral, retains, nevertheless, the typical position of the ball on the back part of the centrum; it is thus biconvex, and the only vertebra of the series which presents that structure.

The first caudal vertebra, moreover, is distinguished from the rest by having no articular surfaces for the hæmapophyses, which in the succeeding caudals form a hæmal arch, like the neurapophyses above, by articulating directly with the centrum. The arch so formed has its base not applied over the middle of a single centrum, but, like the neural arch in the back of the tortoise and sacrum of the bird, across the interspace between two centrums. The first hæmal arch of the tail belongs, however, to the second caudal vertebra, but it is displaced a little backwards from its typical position.

The caudal hæmapophyses, hh, coalesce at their lower or distal ends, from which a spinous process is prolonged downwards and backwards; this grows shorter towards the end of the tail, but is compressed and somewhat expanded antero-posteriorly. The hæmal arch so constituted has received the name of " chevron bone."

It is very true, as Cuvier said in the last lecture he delivered, "if we were agreed as to the crocodile's head, we should be so as to that of other animals; because the crocodile is intermediate between mammals, birds, and fishes." Accordingly, the following description of the crocodile's skull is coextensive with that of the fish; if the answerable bones are rightly determined between these, their correspondence with those of other vertebrates will be facilitated. The difficulties in comprehending the nature of some of the bones of the crocodile's head have arisen through passing to its comparison from that of the mammal's skull-by descending instead of ascending to it.

The segments composing the skull are more modified than those of the pelvis; but just as the vertebral pattern is best preserved in the neural arches of the pelvis, which are called collectively "sacrum," so, also, is it in the same arches of the skull, which are called collectively "cranium. The elements of which these cranial arches are composed preserve, moreover, their primitive or normal individuality more completely than in any of the vertebræ of the trunk, except the atlas, and consequently the archetypal character can be more completely demonstrated.*

If, after separating the atlas from the occiput, we proceed to detach the occipital segment of the cranium from the next segment in advance, we find the detached segment presenting the form and structure of the neural arch. The "centrum" presents, like those of the trunk, a convexity or ball at its posterior articular surface, but its anterior one, like the hindmost centrum of the sacrum, unites with the next centrum in advance by a flat rough "sutural" surface. Like most of the centrums in the neck and beginning of the back, that of the occiput developes a hypapophysis, but this descending

*The skull of the crocodile, partially disarticulated, and with the bones numbered as in the following description, may be had of Mr. Flower, No. 22, Lambeth Terrace, Lambeth Road.

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process is longer and larger, its base extending over the whole of the under surface of the centrum. It is a character whereby the occipital centrum of a crocodilian reptile may be distinguished from that of a lacertian one; for in the latter a pair of diverging hypapophyses project from the under surface, as is shown in most recent lizards and in the great extinct mosasaurus.

The upper and lateral parts of No. 1 present rough sutural surfaces, like those in the centrums of the trunk, for articulating with the "neurapophyses," Nos. 2, 2, which develope short, thick, obtuse, transverse processes, 4, 4. The modified or specialized character of the elements of the cranial vertebræ has gained for them special names. The centrum, 1, is called, as in fishes and all other vertebrates, the "basioccipital;" the neurapophyses, 2, 2, are the "exoccipitals;" the neural spine, 3, is the "superoccipital." The transverse processes, 4, 4, which may combine both diapophyses and parapophyses, are called the "paroccipitals;" they are never detached bones in the crocodilia, as they are in the chelonia and in most fishes. The exoccipitals perform the usual functions of neurapophyses, and, like those of the atlas, meet above the neural canal; they are perforated to give exit to the vagal and hypoglossal nerves, and protect the sides of the medulla oblongata and cerebellum-the two divisions of the epencephalon. The superoccipital, 3, is broad and flat, like the similarly detached neural spine of the atlas; it advances a little forwards, beyond its sustaining neurapophyses, to protect the upper surface of the cerebellum; it is traversed by tympanic air-cells, and assists with the exoccipitals, 2, 2, in the formation of the chamber for the internal

ear.

The chief modification of the occipital segment of the skull, as compared with that of the osseous fish, or with the typical vertebra, is the absence of an attached hæmal arch. We shall afterwards see that this arch is present in the crocodile, although displaced backwards.

66

Proceeding with the neural arches of the crocodile's skull, if we dislocate the segment in advance of the occiput, we bring away, in connection with the long basebone, 5, the bone, 9, which in the figure of the section of the serpent's skull (Cut 17) is shown similarly united to 5. In fact, the centrums of the vertebræ have here coalesced, as we find to happen in the neck of the siluroid fishes, and in the sacrum of birds and mammals. The two connate cranial centrums must be artificially divided, in order to obtain the segments distinct to which they belong. The hinder portion, 5, of the great base-bone, which is the centrum of the parietal vertebra, is called "basisphenoid." It supports that part of the "mesencephalon," which is formed by the lobe of the third ventricle, and its upper surface is excavated for the pituitary prolongation of that cavity. The basisphenoid developes from its under surface a hypapophysis," " which is suturally united with the fore part of that of the basioccipital, but extends further down, and is similarly united in front to the "pterygoids," 24. These rough sutural surfaces of the long descending process of the basisphenoid are very characteristic of that centrum, when detached, in a fossil state. The neurapophyses of the parietal vertebra, 6, 6, or the "alisphenoids," protect the sides of the mesencephalon, and are notched at their anterior margin, for a conjugational foramen transmitting the trigeminal nerve. As accessory functions they contribute, like the corresponding bones in fishes, to the formation of the ear-chamber. They have, however, a little retrograded in position, resting below in part upon the occipital centrum, and supporting more of the spine of that segment, 3, than of their own, 7. The spine of the parietal vertebra is a permanently distinct, single, depressed bone, like that of the occipital vertebra; it is called

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the "parietal," and completes the neural arch, as its crown or key-bone; it is partially excavated by the tympanic air-cells, and overlaps the superoccipital. The bones, 8, 8. wedged between 6 and 7, manifest more of their diapophysial character than their homotypes, 4, 4, do in the occipital segment, since they support modified ribs, are developed from independent centres, and preserve their individuality. They form no part of the inner walls of the cranium, but send outwards and backwards a strong transverse process for muscular attachment. They afford a ligamentous attachment to the hæmal arch of their own segment, and articulate largely with the pleurapophyses, 28, of the antecedent hæmal arch, whose more backward displacement, in comparison with its position in the fish's skull, is well illustrated in the metamorphosis of the toad and frog.

On removing the neural arch of the parietal vertebra, after the section of its confluent centrum, the elements of the corresponding arch of the frontal vertebra present the same arrangement. The compressed produced centrum has its form modified like that of the vertebral centrums at the opposite extreme of the body in many birds; it is called the "presphenoid." The neurapophyses, 10, 10, articulate with the upper part of 9; they are expanded, and smoothly excavated on their inner surface to support the sides of the large prosencephalon; they dismiss the great optic nerves by a notch. They show the same tendency to a retrograde change of position as the neighbouring neurapophyses, 6; for though they support a greater proportion of their proper spine, 11, they also support part of the parictal spine, 7, and rest, in part, below upon the parietal centrum, 5: the neurapophyses, 10, 10, are called "orbitosphenoids." The neural spine, 11, of the frontal vertebra retains its normal character as a single symmetrical bone, like the parietal spine which it partly overlaps; it also completes the neural arch of its own segment, but is remarkably extended longitudinally forwards, where it is much thickened, and assists in forming the cavities for the eye-balls; it is called the "frontal" bone.

In contemplating in the skull itself, or such side view as is given in Fig. 9, p. 22, of my work on the Archetype Skeleton, the relative position of the frontal, 11, to the parictal, 7, and of this to the superoccipital, 3, which is overlapped by the parietal, just as itself overlaps the flattened spine of the atlas, we gain a conviction which cannot be shaken by any difference in their mode of ossification, by their median bipartition, or by their extreme expansion in other animals, that the above-named single, median, imbricated bones, each completing its neural arch, and permanently distinct from the piers of such arch, must repeat the same element in those successive arches-in other words, must be "homotypes," or serially homologous. In like manner the serial homology of those piers, called "neurapophyses," viz., the laminæ of the atlas, the exoccipitals, the alisphenoids, and the orbitosphenoids, is equally unmistakable. Nor can we shut out of view the same serial relationship of the paroccipitals, as coalesced diaphophyses of the occipital vertebra, with the mastoids 8, and the postfrontals, 12, as permanently detached diapophyses of their respective vertebræ. All stand out from the sides of the cranium, as tranverse processes for muscular attachment; all are alike autogenous in the turtles; and all of them, in fishes, offer articular surfaces for the ribs or hæmal arches of their respective vertebræ; and these characters are retained in the postfrontals as well as in the mastoids of the crocodiles.

The frontal diapophysis, 12, is wedged between the back part of the spine, 11, and the neurapophysis, 10; its outwardly projecting process extends also backwards, and joins that of the succeeding diapophysis, 8; but, notwithstanding the retrogradation of

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the inferior arch, it still articulates with part of its own pleurapophysial element, 28, which forms the proximal element of that arch.

There finally remain in the cranium of the crocodile, after the successive detachment of the foregoing arches, the bones terminating the fore-part of the skull; but, notwithstanding the extreme degree of modification to which their extreme position subjects them, we can still trace in their arrangement a correspondence with the vertebrate type.

A long and slender symmetrical grooved bone, 13, between 24 and 24, like the ossified inferior half of the capsule of the notochord, is continued forwards from the inferior part of the centrum, 9, of the frontal vertebra, and stands in the relation of a centrum to the vertical plates of bone, 14, which expand as they rise into a broad, thick, triangular plate, with an exposed horizontal superior surface. These bones, which are called "prefontals," stand in the relation of "neurapophyses" to the rhinencephalic prolongations of the brain commonly but erroneously called "olfactory nerves ;" and they form the piers or haunches of a neural arch, which is completed above by a pair of symmetrical bones, 15, called “nasals," which I regard as a divided or bifid neural spine.

The centrum of this arch is established by ossification in the expanded anterior prolongation of the fibrous capsule of the notochord, beyond the termination of its gelatinous axis. The median portion above specified retains most of the formal characters of the centrum; but there is a pair of long, slender, symmetrical ossicles, which, from the seat of their original development, and their relative position to the neural arch, must be regarded as also parts of its centrum. And this ossification of the element in question from different centres will be no new or strange character to those who recollect that the vertebral body in man and mammalia is developed from three centres. The term "vomer" is applied to the pair of bones, 13, because their special homology with the single median bone, so called in fishes and mammals, is indisputable; but a portion of the same element of the skull retains its single symmetrical character in the crocodile, and is connate with the enormous pterygoids, 24, between which it is wedged. In some alligators (all. niger) the divided anterior vomer extends far forwards, expands anteriorly, and appears upon the bony palate.

Almost all the other bones of the head of the crocodile are adjusted so as to constitute four inverted arches. These are the hæmal arches of the four segments or vertebræ, of which the neural arches have been just described. But they have been the seat of much greater modifications, by which they are made subservient to a variety of functions unknown in the hæmal arches of the rest of the body. Thus the two anterior hæmal arches of the head perform the office of seizing and bruising the food; are armed for that purpose with teeth: and, whilst one arch is firmly fixed, the other works upon it like the hammer upon the anvil. The elements of the fixed arch, called " "maxillary arch," have accordingly undergone the greatest amount of morphological change, in order to adapt that arch to its share in mastication, as well as for forming part of the passage for the respiratory medium, which is perpetually traversing this hæmal canal in its way to purify the blood. Almost the whole of the upper surface of the maxillary arch is firmly united to contiguous parts of the skull by rough or sutural surfaces, and its strength is increased by bony appendages, which diverge from it to abut against other parts of the skull. Comparative anatomy teaches that, of the numerous places of attachment, the one which connects the maxillary arch by its element, 20, with the centrum, 13, and the descending plates of the neurapophyses, 14, of the nasal segment,