14 VERTEBRATE SKELETON 



slightly ossified. Urodeles, like most Anura, lack scales, but a few 

 of the latter order (Ceratophrys, Brachyocephalus) have plates of bone 

 in the skin of the back, sometimes ankylosed to the spinous processes 

 of the vertebrae. Possibly these are coenogenetic. 



REPTILIA. — Most reptiles have a more or less developed exo- 

 skeleton which may be divided into two groups of structures, the 

 dermal scales developed in the corium, and the deeper gastralia. 

 Dermal scales were lacking in Ichthyosaurs (except a few ossicles 

 on the margin of the dorsal fin), Sauropterygia, Pythonomorphs, 

 snakes,^ most lizards and Dinosaurs and all Pterosaurs. When 

 present, the scales are developed in the skin, and in the living species 

 are usually covered by a horny cuticle, mesenchyme intervening 

 between the two. They are usually isolated plates, but frequently 

 they are articulated into a nearly complete armor, represented in the 

 modern Crocodilia by rows of plates embedded in the skin of the 

 back (in Jacare and Caiman of the belly also). In some extinct 

 Crocodiha they overlapped like fish scales, in others they were large 

 articulated plates over the whole body. Many skinks and some 

 Ascalabotse have ossifications in the skin, remnants of the ancestral 

 armor, but most lizards and Sphenodon lack dermal scales. 



Of most of the extinct groups little need be said, except that some 

 of the Stegosaurian Dinosaurs had, not only scattered ossicles, but 

 enormous plates and spines — some plates over a yard across — 

 extending as an enormous comb the length of the back, while Pola- 

 canthus had the lumbar region enclosed in a complete coat of fused 

 scales. 



Most species of Chelonia have the dermal skeleton greatly 

 developed, forming, with the ribs, a case in which, in some genera, 

 head and tail may be retracted. This case consists of a dorsal cara- 

 pace and a ventral plastron, the two sometimes united at the sides 

 by ligament, sometimes this has ossified and carapace and plastron 

 are firmly united by suture or fusion. When fully developed the 

 carapace (fig. 13, .4) consists, besides of ribs, of a series of median 

 ossicles. The most anterior of these, the nuchal plate, is free from 

 the axial skeleton, while the eight following neural plates are fused 

 to the spinous processes of the underlying vertebrae. The median 

 series is completed behind by one or more pygal plates, much like 

 the nuchal. Each neural plate is joined on either side by a costal 



1 The 'scales' of snakes are purely epidermal structures. 



