46 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



also have well-developed gastralia. These are rods of dermal bone in the ventral 

 body wall between the true ribs and the pelvis, which so closely resemble the 

 true ribs that they have been called 'abdominal ribs.' They do not meet in 

 the middle line; each, except the first, consists of two parts and the pairs corre- 



FiG. 37. — Section through developing vertebra, rib and exoskeleton of Chelone 

 imbricata, after Gotte. c, corium in which the dermal plates are developed; cs, primitive 

 vertebral body, ep, epidermis; m, external oblique muscle; p, perichondrium; r, rib; 

 sp, spinous process. 



spond to the somites in number. In Sphenodon (fig. 37) the gastralia are more 

 numerous than the somites. 



In a few lizards there are dermal scales, while the extinct stegosaurs had der- 

 mal ossicles, sometimes of great size (plates a yard across, spines half a yard long) 

 in the dorsal region. 



Fig. 38. — Plastron of Trionyx. en, ento- 

 plastron; ep, epiplastron; hyp, hypoplastron; 

 hy, hyoplastron; xp, xiphiplastron. 



Fig. 39. — Ventral ends of'ribs (r) 

 and gastralia {g) of Sphenodon. 



BIRDS. — All recent birds lack dermal ossifications, but Archaopteryx had 

 gastralia. 



MAMMALS rarely have dermal bones. They are known in the extinct zeuglo- 



