42 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



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

 in the dorsal region. 



BIRDS. Recent birds lack all dermal ossifications, but Archceopteryx had 

 gastralia. 



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

 dont whales and in several fossil edentates, but in the living species they occur 



FIG. 34. Plastron of Trionyx. en, ento- 

 plastron; ep, epi plastron; hpp, hypoplastron; 

 hyp, hyoplastron; xp, xiphiplastron. 



FIG. 35. Ventral ends of ribs (r) 

 and gastralia (g) of Sphenodon. 



only in the armadillos where they form a complete armor above, the plates 

 arranged- in transverse rows, some of which are movable on each other. In the 

 extinct glyptodons they formed an inflexible case. It is uncertain whether these 

 are a new acquisition in the edentates or have been inherited from non-mammalian 

 ancestors. 



THE ENDOSKELETON. 



The endoskeleton may pass through three stages in its develop- 

 ment, including the membranous stage. From this it may pass through 

 a cartilage stage before becoming bone, or it may in part develop 

 directly into bone from membrane, or, lastly, it may never pass beyond 

 the cartilage stage. Thus only the membranous stage is constant. 

 These differences in development are of great importance in tracing 

 homologies between bones in different groups, but the distinction be- 

 tween bones developing directly from membrane (membrane bones) 



