Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 TEETH OF VERTEBEATA. 551 In the Selachii they are partly movable, and are arranged in rows of different ages. In most Fishes they retain their superficial position, and, where they are more firmly united, this is effected by their fusing with the bones which carry them. This is the case also in the Amphibia, where the earliest dental structures form their proper bones by fusing together at their bases. In the Reptilia the teeth are formed independently, like the later teeth of the Am- p
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 TEETH OF VERTEBEATA. 551 In the Selachii they are partly movable, and are arranged in rows of different ages. In most Fishes they retain their superficial position, and, where they are more firmly united, this is effected by their fusing with the bones which carry them. This is the case also in the Amphibia, where the earliest dental structures form their proper bones by fusing together at their bases. In the Reptilia the teeth are formed independently, like the later teeth of the Am- phibia; sometimes they are mere excrescences (pleurodont Lizards); sometimes the developing teeth are sunk into their proper bones. In some of the Saurii, the teeth are attached to the edge of the jaw (acrodont Lizards). In the Geckos and Ophidii, and in all Crocodilini, the developing teeth are partly surrounded by the edges of the jaws, and are, therefore, embedded in alveoli. A similar arrangement obtains in the Mammalia. A mass of epithelium grows into the mucous mem- brane of the edge of the jaw, and forms a cap around a papilla, on which the rudiment of the tooth is developed; as this follicular structure is surrounded by the jaw the tooth is completely differentiated within the jaw, and only breaks through the mucous membrane as it is gradually developed; the saccule which forms it is nipped off from this mucous membrane. The teeth vary very greatly in form, so that there is every intermediate stage between broad plate-like struc- tures, and long, fine, spicular forms; this variety of character is most common in Fishes. The teeth of the Amphibia are more similar in form; in the extant members of the group, at any rate, they are generally simply conical, or faintly notched. Among the Reptilia greater differences are seen in the Saurii, and partly too in the Ophidii, in some of which a certain number of teeth are connected with a special poison apparatus. In the Crocodilini
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