220 



COMPARATIVE ANATOMY 



Fig. 177. — Membrane bones of typical tetrapod; chondrocranium in dotted outline. 

 inter p, interparietal; pftix, premaxilla; pof, postf rental; porb, postorbital; prf, prefrontal; 

 qj, quadratojugal; zyg, zygomatic. (From Kingsley's "Comparative Anatomy of 

 Vertebrates.") 



sDENTARY 

 BONE 



C. . D. 



Fig. 178. — The diagrams A-D illustrate the growth and enlargement of the mem- 

 brane bones of the skull and their encroachment upon the chondrocranium. Cartilage 

 and pro-cartilage bones are stippled, membrane bones black. Membrane bone is 

 represented in the basal plates of the placoid scales of elasmobranchs (A). In the 

 ganoid (B) the scales have fused and enlarged to form bony skutes, but the chondro- 

 cranium remains cartilaginous. In amphibia (C) the cartilage is largely changed to 

 bone and the two kinds of bone become fused together. In mammals (D) very little 

 cartilage is left and the two kinds of bone unite to form bone complexes. Most of the 

 covering bones of the mammalian cranium are membrane bones. 



