xni PHYLUM CHORDATA 24t> 



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

 become segmented into vertebrae. 



The skull (Figs. 870 and 871) 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 supports the tongue and is the sole repre- 

 sentative of the entire visceral or gill-bearing skeleton of Fishes. 



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

 from the skull without injury to the underlying choridrocraniuin. 

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

 alone, but, as in Holocephali and Dipnoi, the primary cranium 

 plus the palato-quadrate or primary upper jaw. The cranium in 

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

 olfactory capsules : the palato-quadrate {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. 118), being a slender rod attached to the cranium at either end, 

 but free in the middle. It is divisible into three regions, a pos- 

 terior quadrate region or suspensorium (Fig. 870, 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 process (Fig. 871, 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 

 paired 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) ; the remaining ossifications of the auditory region (p. 72) 

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

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

 the suspensorium, a small aperture, the fenestra ovalis, closed in the 

 entire animal by membrane, and, when the latter is removed, 



