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 dermal bones distributed over the whole 

 integument, which form a kind of carapace ; in the Scincoidea we 

 generally meet with interlocking bony plates in the integument. 

 Similar kinds of dermal ossifications in the Chelonii form a special, 

 though well-developed form of, dermal skeleton, in consequence 



Fig. 220. Head of Acipenser 

 sturio; seen from above, to show 

 the osseous plates covering the 

 cartilaginous cranium, which is 

 shaded dark. 



