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-qitadratum, o) and the 



Fig. 266. — Head skeleton of a I'limitivt^ Fish: n, nose. groove; eth,, regioL 

 of the sieve-bone ; orh, eye-cavity ; la, wall of eai-labyriuth ; occ, occipital 

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

 cartilage ; 0, primitive upper jaw (palato quadrcttum) ; u, primitive lower 

 jaw ; II., tongue-arch ; III.-VIII., first to sixth gill-arohes. (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.-VIII.). 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 skuU " of the Primitive 

 Fishes originally develops from an equal number (nine at 

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

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

 upper vertebral arches. The coalescence an<l {una Igamation 

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



