260 ZOOLOGY SECT. 



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 

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

 p. 71), which, in this region of the vertebral column, does not 

 become segmented into vertebrae. 



The skull (Figs. 933 and 934) consists of a narrow brain-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 Jiyoid apparatus, which lies in the floor of the mouth 

 and is the sole representative in the skull of the entire hyo- 

 branchial 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 Holocephali 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 (pal. 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. 123), 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 intermediate 

 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 process (Fig. 934, 

 ot. pr) being fused with the auditory capsule, the other the 

 pedicle (ped) with the trabecular region immediately anterior to 

 the auditory capsule. Ventrally 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 pro-otic 

 (PR. OT) ; there are no other ossifications of the auditory 

 region (p. 77). In the adult the pro-otic fuses with the ex- 

 occipital : it presents on its outer surface, behind the otic process 



