Thus, with reference to the main and funda mental tissue of tooth, we find not fewer than six leading modifications in fishes ; —hard or true dentine (Sparoids, Labroids, Lophius, Ba tistes, Pycnodonts, P rionodon, Sphyrwna, Mega liehlhys, Rhizodus, Diodon, Seams) ; osteo dentine (Cestracion, Acrodus, Lepidosiren, Ctenodus, Hybodus, Percoids, Scicenoids, Cot toids, Gobioids, and many others); vaso-den tine (Psammodzes, Chinmeroids, Pristis, Myliobates) ; plici-dentine (Lophius, Holoptychius, Lepidosteus aryurus, at the base of the teeth) ; labyrintho-dentine (Lepidosteus platyrhinus, Bothriolepis); and dendro-dentine (Dendro dus); besides the compound teeth of the Scares 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 persis tency 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 (Scarf), consists of a stratum of prismatic denticles, standing al most vertically to the external surface of the jaw-bone. An account of the structure and development of this peculiar armature of the jaws is abridged from my " Odontography " (pp. 1 I 2-116.),in the article PISCES (VOL I p. 979.). It is peculiarly adapted to the ha bits and exigencies of a tribe of fishes which browse upon the lithoph3tes 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 retract, when touched, into their star shaped stony shells, and the Scan conse quently require a dental apparatus strong enough to break off or scoop out these cal careous recesses. The jaws are, therefore, prominent, short, and stout, as shown in fig. 515. p.979. Vol. III.; and the exposed portions of the premaxilliaries and premandibulars are encased by the complicated dental covering representedin figs. 515 and 516. Vol. III.
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.
The superior dentigerous pharyngeals (figs. 55S. and 565.) present the form of an elon gated, vertical, inequilateral, triangular plate ; the upper and posterior margin is sharp and concave; the upper and anterior margin forms a thickened articular surface, convex from side to side, and playing in a corresponding groove or concavity upon the base of the skull ; the inferior boundary of the triangle is the longest, and also the broadest ; it is convex in the antero-posterior direction, and flat from side to side. It is on this surface that the teeth
are implanted, and in most species they form two rows ; the outer one consisting of very small, the inner one of large dental plates, which are set nearly transversely across the lower surface of the pharyngeal bone, and are in close apposition, one behind the other : their internal angles are produced beyond the margin of the bone, and interlock with those of the adjoining bone when the pharyngeals are in their natural position ; the smaller denticles of the outer row are set in the ex ternal interspaces of those of the inner row.
The single inferior pharyngeal bone con sists principally of an oblong dentigerous plate, of the form represented in fig. 3, pl. 51, of my " Odontography ; " its breadth somewhat exceeds that of the conjoined dentigerous surface of the pharyngeals above, and it is excavated to correspond with their convexity. This dentigerous plate is prin cipally supported by a strong, slightly curved, transverse, osseous bar, the extremities of which expand into thick obtuse processes for the implantation of the triturating muscles. A longitudinal crest is continued downwards and forwards from the middle line of the inferior pharyngeal plate, anterior to the transverse bar, to which the protractor mus cles are attached.
A longitudinal row of small oval teeth alternating with the large lamelliform teeth, like those of the superior pharyngeals, bounds the dentigerous plate on each side; the inter mediate space is occupied exclusively by the larger lamelliform or wedge-shaped teeth, set vertically in the bone, and arranged trans versely in alternate and pretty close set series.
The dental plates are developed in wide and deep cavities in the substance of the posterior part of the lower, and of the an terior part of the upper pharyngeal bones. Each denticle is developed in its proper cap sule, which contains an enamel-forming pulp and a dentinal pulp, in close cohesion with each other and with the thin external capsule. The teeth exhibit progressive stages of form ation as they approach the posterior part of the upper and the anterior part of the lower pharyngeal bones : as their formation advances to completion they become soldered together by ossification of their respective capsules into one compound tooth, which soon be conies anchylosed by ossification of the den tinal pulp to the pharyngeal bone itself.