296 THE EVOLUTION OF MAN. 



vertebrae," in the selachian skull (Fig. 2G5), the following 

 pairs of arches : I. and II. are two lip cartilages, of which 

 the anterior (a) consists only of an upper, and the inferior 

 (be) of an upper and a lower piece; III., the jaw-arch, 

 which also consists of two pieces on each side, viz., the 

 primitive upper jaw (os palato-quadratum, o) and the 



FIQ. 265. Head skeleton of a Primitive Fish: u, uose-groove; eth, region 

 of the sieve-bone ; orb, eye-cavity ; la, wall of ear-labyrinth ; occ, occipital 

 region of the primitive skull ; cv, vertebral column ; a, front ; be, hind lip- 

 cartilage ; o, primitive upper jaw (palato quadraturii) ; u, primitive lower 

 jaw; 11., tongue-arch ; II1.-V11I., first to sixth gill-arches. (After Gegen- 

 baur.) 



primitive lower jaw (it); IV., the tongue arch (II.), and V. to 

 X., six true gill arches, in the stricter sense of that term 

 (1II.-VIIL). The anatomical features of these nine or ten 

 skull-ribs, or " lower vertebral arches," and of the brain 

 nerves distributed over them, show that the apparently 

 simple, cartilaginous "primordial skull" of the Primitive 

 Fishes originally develops from an equal number (nine at 

 the least) of primitive vertebrae. The base of the skull is 

 formed by the vertebral bodies ; the roof of the skull by the 

 upper vertebral arches. The coalescence and amalgamation 

 of these into a single capsule is, however, so ancient, that 



