268 Z(^OLOGY rkct. 



of the skull (vide infra). The eiglith vertebra has a biconcave 

 centrum ; that of the ninth or sacral vertebra, (v. 9) is convex in 

 front and presents posteriorly a double convexity articulating with 

 a double concavity on the anterior end of the urostyle. The latter 

 (a. ST) is formed by the ossification of the perichordal tube (see 

 p. 73) which, in this region of the vertebral column, does not 

 become sea^mented into vertebra?. 



The shull (Figs. 922 and 923) consists of a narrow hrain-case, 

 produced behind into great outstanding auditory capsules, and in 

 front into large olfactory capsules. The whole of the bones of the upper 

 jaw are immovably fixed to the cranium, so that the only free parts 

 are the lower jaw and a small plate of mingled bone and cartilage, 

 the hyoid apparatus, which lies in the floor of the mouth and is the 

 sole representative in the skull of the entire visceral or gill- 

 bearing skeleton of Fishes. 



As in the Trout, a number of investing bones can be removed 

 from the skull without injury to the underlying chondrocranium. 

 The latter, however, is not, as in the Trout, the primary cranium 

 alone, but, as in Holocej^hali and Dipnoi, the primary cranium 

 plus the palatoquadrate or primary upper jaw. The cranium in 

 the strict sense includes the brain-case and the auditory and 

 olfactory capsules : the palatoquadrate (pcd. qu) is not a solid mass 

 fused throughout its length with the cranium, as in Holocephali and 

 Dipnoi, but rather resembles the subocular arch of the Lamprey 

 (p. 120), being a slender rod attached to the cranium at either 

 end, but free in the middle. It is divisible into three regions, 

 a posterior quadrate-region or suspensorium (sus), an inter- 

 mediate pterygoid region, and an anterior palatine region. The 

 suspensorium extends backwards, outwards, and downwards from 

 the auditory region of the cranium, to which it is immovably 

 united by its forked proximal end, one branch of the fork — 

 the otic ptrocess (Fig. 923, ot. pr) — being fused with the auditory 

 capsule, the other — the p)edicle {peel) — with the trabecular region 

 immediately anterior to the auditory capsule. A^entrally the 

 suspensorium furnishes an articular facet for the mandible, and is 

 connected with the delicate rod-like pterygoid region ; this passes 

 forwards and joins the palatine region, which is a transverse bar 

 fused at its inner end with the olfactory capsule. 



The occipital region of the cranium contains only two bones, 

 the exoccipitals (ex. oc), which lie one on each side of the foramen 

 magnum {for. mag) and meet above and below it : there is no 

 trace of supra- or basi-occipital. Below the foramen magnum are 

 a pair of oval projections, the occipital condyles (oc. en), furnished by 

 the exoccipitals and articulating with the cervical vertebra. 



Each auditory capsule is ossified by a single bone, the p)ro-otic 

 (PR. ot) ; the remaining ossifications of the auditory region (p. 79) 

 are not developed. In the adult the pro-otic fuses with the exoc- 



