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IV. "Osteology of Polyodon folium." By T. W. BRIDGE, B.A., Scholar of Trinity College, and Demonstrator of Comparative Anatomy in the University of Cambridge. Communicated by Professor NEWTON, F.R.S. Received May 29,

1878.

(Abstract.)

On a previous occasion* the author described the cranial osteology of one of the more specialized genera of existing Ganoids, and the paper now laid before the Society is an attempt to add to the relatively scanty literature of this department of comparative anatomy, by giving in detail an account of the skeletal structures of one of the most generalized members of the group. The following are among the more important of the conclusions which the facts elucidated appear to warrant.

A comparison of Polyodon with its nearest living ally, the Sturgeon, demonstrates the close relationship that exists between the two forms as regards their skeletal anatomy. In so far as Acipenser differs from Polyodon, it approaches the Teleostean type, nor is the latter genus without indications of having undergone a degree of specialization in the same direction. But not the least important characteristic of Polyodon is its possession of a remarkable combination of structures, usually regarded as being specially distinctive of the Plagiostome Elasmobranchii. The existence of such primitive characters, which are also present in Acipenser, though to a less extent, suggests the desirability of giving to such facts adequate expression in any scheme that may be proposed for the classification of the Ganoids. After giving a resume of the views held by various zoologists as to the position of these genera in their order, it is suggested that J. Müller's two suborders (Chondrostei and Holostei) should be retained, though it seems advantageous to remodel the definitions on which they were based. The various families of recent Ganoidei may be arranged in two suborders of approximately equal morphological value, which are mainly differentiated by the structure of their upper jaws.

a. G. Selachoidei.-Pterygoid processes united in a medio-ventral symphysis beneath the basis cranii; notochord persistent and unsegmented; spiracles and mandibular branchiæ present.

Gen. Polyodon, Acipenser, Scaphirhynchus, Chondrosteus.

B. G. Teleosteoidei.-Pterygoid processes not united with each other but are connected directly, or indirectly by the intervention of a palatine bone, with the prefrontal region of the skull; vertebral column generally ossified; mandibular gills abortive.

Gen. Amia, Lepidosteus, Polypterus.

* "Journal of Anatomy and Physiology," vol. xi.

If tested by the structure of their upper jaws, the Crossopterygidæ, Lepidosteidæ, and the Palæoniscidae should be referred to the Teleosteoidei, while the Acanthodida would probably belong to the other sub-order. Nevertheless, the distinctness of the two groups is materially lessened by the existence of many annectent fossil forms. Acipenser and Chondrosteus in the one division and Palæoniscus in the other, partially bridge over the gap which exists between the two when the recent forms only are considered. The Placodermi and the Cephalaspidæ must still be referred to as being "incertæ sedis."

A comparison of the skull of Polyodon with the Amphibian skull leads to interesting results. Perhaps the most remarkable feature in which Polyodon resembles the Anura, is in the possession of a forwardly directed "orbitar process" associated with a suspensorium so much inclined backwards that the gape is extended even beyond the posterior limits of the skull.

The condition of the "orbitar process," as an apparently functionless rudiment in Polyodon, does not throw any light on its primitive origin, but its position and relations in the adult Lamprey, and its transitory condition in the embryo Anura, suggest that originally it may have acted as an anterior suspensor to the much inclined mandibular pier of animals possessing a suctorial mouth, prior to the adaptation of the pterygo-quadrate arcade to that purpose. These facts, together with the rotation of the quadrate cartilage, which we may infer to have taken place from the direction and relations of the "orbitar process," are indications of the existence of a close parallelism between the developmental history of the cranium in the embryo Polyodon, and in such otherwise dissimilar Anurous Amphibia as Dactylethra capensis, Bufo ornatus, and Rana temporaria. The fenestration of the roof of the periotic capsule which exists in Polyodon seems to correspond to the primitive auditory involution which persists in Siren lacertina, and, as in the latter, it is situated to the outer side of the arch of the posterior vertical semicircular canal, and not to its inner or mesial side as is the case with the Selachians.

The co-existence in Polyodon of so remarkable a combination of Amphibian and Selachian features suggests an enquiry into the phylogenetic relationship of the Ganoids, the Amphibia, and the Elasmobranchs, or, in other words, suggests the question, is the ancestral stem of the Ganoidei more closely related to that of the Amphibia, or to that of the Elasmobranchs? An analysis of the structural features common to any two of these primary groups seems to affirm the monophyletic origin of the two first-mentioned. On this assumption the relation of the three groups may be roughly and tentatively expressed as follows:

It seems not improbable that a primitive ancestral stock (x) very

early differentiated into the two groups of Apneumatocola and Pneumatocola, the former being the root-stock of the modern Elasmobranchii, while the latter, by acquiring rudimentary and more or less functional lungs, became the primitive double-breathers from which have been derived the Ganoidei and the Amphibia. From the primi. tive Ganoidei were derived the Teleosteoid Ganoids and eventually the Teleostei also, their originally complex swim-bladders becoming gradually devoted to other functions, while the Selachoidei may be regarded as the but little modified descendants of the original progenitors of the order. The close correspondence that exists between Polyodon and the Selachii is not incompatible with these views, but may be the result of the persistence in both of structures originally possessed by their primitive ancestor. Two facts in the cranial anatomy of Polyodon are not easy to explain, viz., the formation of the upper jaw and the existence of the "orbitar process." The union of the pterygoid processes in a median symphysis may have been the primitive condition of the jaws in the ancestral form (x), but that while persistent in Polyodon and in the Selachii, it was superseded by a different arrangement, viz., the union of the pterygoid processes with retral palatine outgrowths in most Ganoidei, and in all Teleostei and Amphibia. Neither is it easy to account for the retention of the " orbitar process." It may have been an adaptive modification correlated with a suctorial mouth in the larval or adult forms of those Ganoids that were first differentiated from the Amphibian stem, and independently developed; or it may have been possessed by, and similarly functioned in, the primitive Pneumatocola, but has become obsolete in all their descendants, except Polyodon and the Anura. Thus it would appear that the Polyodontidae constitute a remarkably central group. They retain not a few of the characters which we may assume to have belonged to the primitive stock out of which were evolved the three most important groups of Ichthyopsida, combined, however, with a certain amount of specialization; nor are they altogether without indications of retrogression.

V. "On Astrophiura permira, an Echinoderm-form intermediate between Ophiuroidea and Asteroidea." By W. PERCY SLADEN, F.L.S., F.G.S. Communicated by Professor DUNCAN, F.R.S. Received June 18, 1878.

(Abstract.)

The following peculiarities of structure presented by the anatomy

of the echinoderm above described are noteworthy:

1. The combination of ophiuroid disk- and arm-structure within a pentagonal asteroid form of body.

2. The asteroid character of the ambulacral system: the divisional plates not only being homologous with, but resembling in the manner of their disposition the ambulacral plates of Asteroidea; at the same time furnishing a highly suggestive representation of their phylogenetic development.

3. The rudimentary structure of the mouth-armature, more asteroid than ophiuroid in general facies. Absence of teeth, jaw-plates, and jaws.

4. Extension of the peritoneal cavity to the extremity of the functional portion of the rays, that is to say, to the margin of the pentagonal body.

5. The extremely rudimentary condition and aborted character of that portion of the brachial series which is prolonged beyond the bodydisk.

6. The continuity of the tentacular pore-system limited to the disk only.

The above characters are clearly sufficient to stamp the peculiarity of this extraordinary echinoderm, and, whilst excluding it from any known group of genera by their remarkable nature and by the aberrant departure they present from all previous types, are such as would seem to necessitate the relegation of the form to a family apart by itself.

To speak definitely as to the exact position of intermediacy which the organism holds between the Asteroidea and Ophiuroidea would obviously be premature, without a more detailed examination of the internal anatomy than the present specimen in its dry condition will permit, as well as some knowledge of the life-history of the form. It may however be safely affirmed without overstepping the bounds of due caution, that Astrophiura bridges further over, from the ophiuroid side, the differences which have separated the two orders, than any previously described starfish or brittle-star.

VI. "Experimental Researches on the Temperature of the Head." Part II, III, IV. By J. S. LOMBARD, M.D., formerly Assistant Professor of Physiology in Harvard University, U.S. Communicated by H. CHARLTON BASTIAN, M.D., F.R.S., Professor of Pathological Anatomy in University College, London. Received June 18, 1878.

(Abstract.)

Part II.-Examination of the Middle Region of the Head.

This region is divided on each side into 7 tiers by 6 equidistant

lines drawn parallel to the longitudinal median line.

The tiers are

VOL. XXVII.

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numbered from 1 to 7 from below upwards. There are 5 districts in each lateral half, formed by 4 equidistant lines drawn parallel to the anterior and posterior boundaries. The districts are numbered fron 1 to 5 from the anterior boundary backward. Each tier measures vertically on the anterior boundary line 22·78 mm. (0·89 inch), and each district measures horizontally on the median line 16.6 mm. (0·65 inch). The ear cuts off the greater portion of the 3rd district, the whole of the 4th district, and a little of the 5th district—all in the 1st tier. The following are the principal results of the examination of the middle region:

1st. Comparison of symmetrically situated spaces of the two sides of the head, 100 observations on each pair of spaces.

As in the case of the anterior region, every space may be higher in temperature on the right side or on the left side in turn. The following is the distribution of temperature in the majority of cases:—

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Thus in 34 spaces a side compared, the temperature is higher on each side in 17 spaces. Taking the total number of observations, 3,400, and deducting 107 cases of equality of temperature, the following are the percentages of times of occurrence of relative superiority of temperature for the right and left sides respectively:

Right side,-49-711. Left side,-50-289.

The following are the spaces in which equality of temperature is most generally found:

1st Tier.

4th Tier.

5th Tier.

Districts-1st, 2nd, 3rd. 2nd, 3rd, 4th, 5th. 2nd, 3rd, 4th, 5th.

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The following are the percentages of superiority of temperature on the right side, on the left side, and of equality of temperature,—in the total number of observations :

Right side-48 147 per cent.

Left side-48.706 per cent.

Equality-3-147 per cent.

For the boundaries of the different regions, see Abstract, Part I.

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