Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 INTEGUMENT OF VERTEBRATA. 425 the fishes, there are others which are of more importance; these are the bony plates which are definitely arranged, and constantly present, on the head, where they form the earliest rudiments of the bony skull, or, at first, of the roof of the skull (cf. Fig. 220). These dermal bones are inherited by all Vertebrata that are pro- vided with a bony skull, and are connected with other ossifications, which do not appear till later, in the carti- laginous skull. Th
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 INTEGUMENT OF VERTEBRATA. 425 the fishes, there are others which are of more importance; these are the bony plates which are definitely arranged, and constantly present, on the head, where they form the earliest rudiments of the bony skull, or, at first, of the roof of the skull (cf. Fig. 220). These dermal bones are inherited by all Vertebrata that are pro- vided with a bony skull, and are connected with other ossifications, which do not appear till later, in the carti- laginous skull. This arrangement is first seen in the Sturiones. There are a number of smaller bony plates in addition to the large ones, but most of these have no general sig- nificance. On account of these rela- tions to the internal skeleton, their more special characters will be ex- pounded when we come to treat of it. Other skeletal parts besides the bones of the skull are derived from ossifica- tions of the integument; the clavicle, for example, has a similar origin. Lastly, there is another category of bones which are likewise derived from placoid scales; the bones around the mouth have been recognised as having their origin in tooth-bear- ing plates derived from fused placoid scales. § 320. We meet with dermal bones in the higher classes; in the Amphibia, and also in the fossil Archegosaurii, in which there were dermal bones in the form of scutiform plates. We find only scattered dermal bones in a ru- dimentary form in extant Amphibia. In Ceratophrys there is an osseous shield in the skin of the back; in Brachycephalus there are three which are united to several vertebra?. The bony scales which are pretty generally found in the Coccilia?, and which are set in pouch-shaped depressions, do not appai'ently belong to this set of structures. They are more common in the Reptilia, which so far approach the old Amphibian phylum. In the fossil Teleosaurii, as in the living Crocodilini, there are
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