i6 



VERTEBRATE SKELETON 



There is great uncertainty as to the homologies of these plates in the Chelon- 

 ian skin. The ossicles of the Dermochelydae certainly belong to the dermal skele- 

 ton, and the same probably holds for marginals, nuchals and pygals. Costals and 

 neurals are held by some to belong to the axial skeleton. The plastral bones are 

 supposed to be derived from the ventral scales of some Stegocephal-like form, 

 but no intermediate stages are known. The possible homology of entoplastron 

 and epiplastra with episternum and clavicles will be mentioned later. 



In Crocodilia and Sphenodon of living forms, and in the extinct 

 Rhynchocephals, Ichthyosaurs, Plesiosaurs, Phytosaurs, Thalatto- 

 saurs, some Dinosaurs and all Pterosaurs another 

 series of dermal bones, the gastralia, occur; they 

 are also called abdominal ribs and parastema. 

 They are rib-like in appearance and occupy the 

 gap between the last true rib and the pelvis. 

 They lie in the rectus muscles of the abdomen, 

 are not connected with the vertebrae and in their 

 development no cartilage appears. They are 

 composed of two or more overlapping bones on 

 either side, those of the two sides converging 

 forwards. The number varies from one to a 

 somite in Crocodiha (fig. 14), two in Sphenodon, 

 up to six in some fossils. These facts all militate 

 against their being ribs; the usual interpretation is 

 that they are the representatives of the abdominal 

 armor of Stegocephals (p. 13). 



AVES.^ — With the exception of the ancient 

 ArcJiaopteryx. dermal bones are unknown among 

 Aves. Archcsopteryx has gastralia in the abdominal 

 walls, but no other dermal bones. 



MAMMALIA. — Gastralia are lacking in all 

 mammals, and dermal bones occur only in Eden- 

 tates and Cetacea. Some Edentates have an 

 armor of dermal scales around the body, arranged 

 either in transverse rows (armadillos) or as a 

 mosaic of polygonal plates {Glyptodon), the ossicles 

 in both extending on head, limbs and tail. In 

 armadillos the scales arise from isolated centres 

 in the corium, these meeting later, with partial 

 obhteration of the hair. The arrangement differs in the several 

 genera. 



Fig. 14. — G a s- 

 tralia, pelvis and part 

 of sternum of Croc- 

 odiliis ( Voeltzkow und 

 Doderlein, 'oi); carti- 

 age stippled, bones 

 lo ut lined, g, gas- 

 tralia; i, ischium; p, 

 pubis; r, ribs; s, 

 sternum. 



