TEETH OF THE BARRACUDA FISH. $273 Osteodentine - Cestracion, acrodus, lepidosiren, ctenodus, hybodus, percoids, scianoids, cottoids, gobioids, sharks, and many others; Vasodentine Psammodus, chimæroids, pristis, myliobates; Besides the compound teeth of the scarus and diodon. One structural modification may prevail in some teeth, another in other teeth, of the same fish; and two or more modifications may be present in the same tooth, arising from changes in the process of calcification and a persistency of portions or processes of the primitive vascular pulp or matrix of the dentine. The dense covering of the beak-like jaws of the parrot-fishes (Scari) consists of a stratum of prismatic denticles, standing almost vertically to the external surface of the jaw bone; this peculiar armature of the jaws is adapted to the habits and exigencies of a tribe of fishes which browse upon the lithophytes that clothe, as with a richly tinted carpet, the bottom of the sea, just as the ruminant quadrupeds crop the herbage of the dry land. The irritable bodies of the gelatinous polypes which constitute the food of these fishes' retreat, when touched, into their star-shaped stony shells, and the scari consequently require a dental apparatus strong enough to break off or scoop out these calcareous recesses. The jaws are, therefore, prominent, short, and stout, and the exposed portions of the premaxillaries and premandibulars are incased by a complicated dental covering. The polypes and their cells are reduced to a pulp by the action of the pharyngeal jaws and teeth that close the posterior aperture of the mouth. There is a close analogy between the dental mass of the scarus and the complicated grinders of the elephant, both in form, structure, and in the reproduction of the component denticles in horizontal succession. But in the fish, the complexity of the triturating surface is greater than in the mammal, since, from the mode in which the wedge-shaped denticles of the scarus are implanted upon, and anchylosed to, the processes of the supporting bone, this likewise enters into the formation of the grinding surface when the tooth is worn down to a certain point. The proof of the efficacy of the complex masticatory apparatus above described, is afforded by the contents of the alimentary canal of the scari. Mr. Charles Darwin, the accomplished naturalist and geologist, who accompanied Captain Fitzroy, R.N., in the circumnavigatory voyage of the "Beagle,” dissected several parrot-fishes soon after they were caught, and found the intestines laden with nearly pure chalk, such being the nature of their excrements; whence he ranks these fishes among the geological agents to which is assigned the office of converting the skeletons of the lithophytes into chalk. The most formidable dentition exhibited in the order of osseous fishes is that which characterizes the sphyræna, and some extinct fishes allied to this predatory genus. In the great barracuda of the southern shores of the United States (Sphyræna barracuda, Cuv.) the lower jaw contains a single row of large, compressed, conical, sharp-pointed, and sharp-edged teeth, resembling the blades of lancets, but stronger at the base; the two anterior of these teeth are twice as long as the rest, but the posterior and serial teeth gradually increase in size towards the back part of the jaw; there are about twenty-four of these piercing and cutting teeth in each premandibular bone. They are opposed to a double row of similar teeth in the upper jaw, and fit into the interspace of these two rows when the mouth is closed. The outermost row is situated on the 274 DENTAL SYSTEM OF REPTILES. intermaxillary, the innermost on the palatine bones; there are no teeth on the vomer or superior maxillary bones. The two anterior teeth in each premaxillary bone equal the opposite pair in the lower jaw in size; the posterior teeth are serial, numerous, and of small size; the second of the two anterior large premaxillary teeth is placed on the inner side of the commencement of the row of small teeth, and is a little inclined backwards. The retaining power of all the large anterior teeth is increased by a slight posterior projection, similar to the barb of a fish-hook, but smaller. The palatine bones contain each nine or ten lancet-shaped teeth, somewhat larger than the posterior ones of the lower jaw. All these teeth afford good examples of the mode of attachment by implantation in sockets, which has been denied to exist in fishes. The loss or injury to which these destructive weapons are liable, in the conflict which the sphyræna wages with its living and struggling prey, is repaired by an uninterrupted succession of new pulps and teeth. The existence of these is indicated by the foramina, which are situated immediately posterior to, or on the inner margin of, the sockets of the teeth in place; these foramina lead to alveoli of reserve, in which the crowns of the new teeth in different stages of development are loosely imbedded. It is in this position of the germs of the teeth that the sphyranoid fishes, both recent and fossil, mainly differ, as to their dental characters, from the rest of the scomberoid family, and proportionally approach the sauroid type. In all fishes the teeth are shed and renewed, not once only, as in mammals, but frequently during the whole course of their lives. The maxillary dental plates of lepidosiren, the cylindrical dental masses of the chimæroid and edaphodont fishes, and the rostral teeth of the saw-fish (if these modified dermal spines may be so called) are, perhaps, the sole examples of "permanent teeth" to be met with in the whole class. In the great majority of fishes, the germs of the new teeth are developed like those of the old, from the free surface of the buccal membrane throughout the entire period of succession; a circumstance peculiar to the present class. The angler, the pike, and most of our common fishes, illustrate this mode of dental reproduction; it is very conspicuous in the cartilaginous fishes (Fig. 8, e. g.), in which the whole phalanx of their numerous teeth is ever marching slowly forwards in rotatory progress over the alveolar border of the jaw, the teeth being successively cast off as they reach the outer margin, and new teeth rising from the mucous membrane behind the rear rank of the phalanx. This endless succession and decadence of the teeth, together with the vast number in which they often co-exist in the same fish, illustrate the law of "vegetative or irrelative repetition," as it manifests itself on the first introduction of new organs in the animal kingdom, under which light we must view the above-described organized and calcified preparatory instruments of digestion in the lowest class of the vertebrate series. Dental System of Reptiles.-In the class reptilia an entire order (Chelonia), including the tortoises, terrapenes, and turtles, are devoid of teeth; but the jaws in these edentulous reptiles are covered by a sheath of horn, which in some species is of considerable thickness and density; its working surface is trenchant in the carnivorous species, but is variously sculptured and adapted for both cutting and bruising in the vegetable feeders. No species of toad possesses teeth; neither have the jaws the compensatory covering above described in the chelonians. Frogs have teeth in the upper but not in the lower jaw. Newts and salamanders have teeth in both jaws, and also upon the palate; and teeth are found in the latter situation as well as on the jaws in most serpents and in the iguana lizard. In most other lizards and in crocodiles the DENTAL SYSTEM OF REPTILES. 275 teeth are confined to the jaws: in the former they are cemented or anchylosed to the jaw; in the latter they are implanted in sockets. Fig. 10. The existing lizards exhibit many modifications in the form of the teeth according to the nature of the food. They are pointed with sharp cutting edges in the great carnivorous monitor (Varanus), and are obtuse and rounded like paving-stones in the herbivorous or mixed feeding scinks, called, on account of the shape of the teeth, cyclodus. The gigantic extinct lizards showed similar modifications of their teeth. The megalosaurus had teeth which combined the properties of the knife, the sabre, and the saw (Fig. 10). When first protruded above the gum, the apex of the tooth presented a double cutting edge of serrated enamel; its position and line of action were nearly vertical, and its form, like that of the two-edged sword, cutting equally on each side. As the tooth advanced in growth it became curved backwards in the form of a pruning-knife, and the edge of serrated enamel was continued downwards to the base of the concave and cutting side of the tooth; whilst on the other side a similar edge descended but a short distance from the point, and the convex part of the tooth became blunt and thick, as the back of a knife is made thick for the purpose of producing strength. In a tooth thus formed for cutting along its concave edge, each movement of the jaw combined the power of the knife and the saw. The backward curvature of the fullgrown teeth enables them to retain, like barbs, the prey which they had penetrated. In the iguanodon-the gigantic contemporary of the Fig. 11. TOOTH OF THE MEGALOSAURUS. megalosaurus-the crown of the teeth (Fig. 11) was so shaped, that after the apex became worn down, A it presented a broad and nearly horizontal surface, exposing dental substances of four different degrees of density,-viz., a ridge of enamel along the outer border of the crown; a layer of hard or unvascular dentine next to this; a layer of softer vascular dentine forming the inner half of the crown; and a portion of firm osteo-dentine in the middle of the grinding surface, formed by the ossified remnant of the tooth-pulp. The series of complex teeth, so constructed, seems to have been admirably adapted to the cropping and comminution of such tough vegetable food as the clathrarie and similar now extinct plants, the fossil remains of which are NEW-FORMED AND WORN TEETH found buried with those of the iguanodon. No existing reptile now presents so complicated a structure of the tooth in relation to vegetable food. The still more complex, and indeed marvellous structure of the teeth of the extinct gigantic lizard-like toad, called Labyrinthodon, has been already noticed (Fig. 4, p. 265). But, perhaps, the most singular dental OF THE IGUANODON. structure yet found in the ancient members of the class Reptilia, is that presented by certain species of fossil found in South Africa, and probably from a geological formation nearly as old as our coal strata. I have called them "Dicynodonts," from their dentition being reduced to one long and large canine tooth on each side of the upper jaw. As these teeth give, at first sight, a character to the jaws like that which the long poison-fangs give, when erected, to the jaws of the rattlesnake, I shall briefly notice their characters before entering upon the description of the more normal saurian dentition. Fig. 12 gives a reduced side view of the skull and teeth of the Dicynodon lacerticeps. Fig. 12. The maxillary bone, 21, is excavated by a wide and deep alveolus, with a circular area of half an inch, and lodges a long and strong, slightly curved, and sharp-pointed, canine tooth or tusk, which projects about two-thirds of its length from the open extremity of the socket. The direction of the tusks is forwards, downwards, and very slightly inwards; the two converging in the descent along the outer side of the compressed symphysis of the lower jaw, cc. The tusk is principally composed of a body of compact unvascular dentine. The base is excavated by a wide conical pulp-cavity, P, with the apex extending to about one-half of the implanted part of the tusk, and a linear continuation extending along the centre of the solid part of the tusk. SKULL AND TUSKS OF Dicynodon lacerticeps. Until the discovery of the rhynchosaurus, this edentulous and horn-sheathed condition of the jaws was supposed to be peculiar to the chelonian order among reptiles; and it is not one of the least interesting features of the dicynodonts of the African sandstones, that they should repeat a chelonian character hitherto peculiar amongst lacertians, to the above-cited remarkable extinct edentulous genus of the new red sandstone of Shropshire; but our interest rises almost to astonishment, when in a saurian skull we find, superadded to the horn-clad mandibles of the tortoise, a pair of tusks, borrowed, as it were, from the mammalian class, or rather foreshadowing a structure which, in the existing creation, is peculiar to certain members of the highest organized warmblooded animals. In the other reptilia, recent or extinct, which most nearly approach the mammalia in the structure of their teeth, the difference characteristic of the inferior and coldblooded class is manifested in the shape, and in the system of shedding and succession of the teeth; the base of the implanted teeth seldom becomes consolidated, never contracted to a point, as in the fangs of the simple teeth of mammalia, and at all periods of growth one or more genus of teeth are formed within or near the base of the tooth in use, prepared to succeed it, and progressing towards its displacement. The dental armature of the jaws is kept in serviceable order by uninterrupted change and succession; but the forming organ of the individual tooth is soon exhausted, and the life of the tooth itself may be said to be comparatively short. TEETH OF CROCODILES AND POISONOUS SNAKES. 277 If one of the conical, sharp-pointed, and two-edged teeth of the Gangetic crocodile, called 66 garrhial" by the Hindoos, be extracted, its base will be found hollow, and partly absorbed or eaten away, as at a, Fig. 13; and within the cavity will be seen the half-formed succeeding tooth, b; at the base of which may probably be found the beginning or germ, c, of the successor of that tooth: all the teeth in the crocodile tribe being pushed out and replaced in the vertical direction by new teeth, as long as they live. The individual teeth increase in size as the animal grows; but the number of teeth remains the same from the period when the crocodile quits the egg to the attainment of its full size and age. No sooner has the young tooth penetrated the interior of the old one, than another germ begins to be developed from the angle between the base of the young tooth and the inner alveolar process, or in the same Fig. 13. TOOTH, WITH GERMS OF SUCCESSORS, OF THE GARRHIAL (Gavialis gangeticus). relative position as that in which its immediate predecessor began to rise; and the processes of succession and displacement are carried on, uninterruptedly, throughout the long life of these coldblooded carnivorous reptiles. The fossil jaws of the extinct croco diles demonstrate that the same law regulated the succession of the teeth at the ancient epochs when they prevailed in greatest numbers, and under the most varied specific modifications, as at the present day, when they are reduced to a single family. The most complex condition of the dental system in the reptile class is that which is presented by the poisonous serpents, in which certain teeth are associated with the tube or duct of a poison-bag and gland. These teeth, called "poison-fangs," are confined to those bones of the upper jaw called "maxillary," and are usually single, or, when more, one only is connected with the poison-apparatus, and the others are either simple teeth, or preparing to take the place of the poison-fang. To give an idea of the structure of this tooth, we may suppose a simple slender tooth, like that of a boa-constrictor, to be flattened, and its edges then bent towards each other and soldered together so as to form a tube, open at both ends, and inclosing the end of the poison-duct. Such a tooth is represented at Fig. 14, where A is the oblique opening penetrated by the duct, and v the narrower fissure by which the venom escapes. The duct which conveys the poison, although it runs through the centre of the tooth, is really on the outside of the tooth. The bending Fig. 14. A of the dentine about it begins a little beyond the base of the tooth, where the poison-duct rests in a slight groove or longitudinal indentation on the convex side of the fang; as it proceeds it sinks deeper into the substance of the tooth, and the sides of the groove meet and seem to coalesce, so that the trace of the inflected fold ceases, in some species, to be perceptible to the naked eye; and the fang appears, as it is commonly described, to be perforated by the duct of the poison-gland. In the viper the line of union may be seen as marked at v, Fig. 14; and when such POISON-FANG OF (Magnified). |