296 THE EVOLUTION OF MAN. 



vertebrae," in the selachian skull (Fig. 265), 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 



Fio. 265. Head skeleton of a Primitiyo Fish: n, nose-groove; eth, region 

 of the sieve-bone ; orb, eye-cavitj ; la, wall of ear-labyriuth ; occ, occipital 

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

 cartilage ; o, primitive upper jaw (palato quadratum) ; it, primitive lower 

 jaw; 11., tongue-arch ; IIL-VI1L, first to sixth gill-arches. (After Gegen- 

 baur.) 



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

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

 (III.-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 



