224 



FISHES 



the whole length of its dorsal border the palato-quadrate carti- 

 lage is fused with the inferior lateral margin of the cranium from 

 the periotic to the olfactory region, thus forming a triangular 

 plate of cartilage, the base of which is continuous with the 

 cranium, while the downwardly directed apex provides an articu- 

 lar surface for the lower jaw. The hyoid arch is little better 

 developed than the succeeding branchial arches, and includes a 

 vestigial hyomandibular, an epi-hyal, and a cerato-hyal. As in 

 other autostylic skulls the hyomandibular element is attached by 

 ligament to the hinder margin of the palato-quadrate, instead 



FIG. 130. Side view of the skull of a Sturgeon, with the investing membrane bones re- 

 moved, a, Pharyngo-branchial ; AF, antorbital or lateral ethmoid cartilage ; AR, 

 articular ; b, epi-branchial ; c, cerato-brauchial ; C, notochord ; Cop, basi-brauchials ; 

 d, hypo-branchial ; De, dentary ; GK, auditory capsule ; ffm, hyomandibular ; hy, 

 cerato-hyal ; Ih, inter-hyal ; Md, lower jaw ; Xa, nasal capsule ; Ob, neural arches ; 

 Orb, Orbit ; PF, post-orbital process ; PQ, palato-quadrate ; Ps, Ps', Ps", para- 

 sphenoid ; Psp, neural spines ; Qu, quadrate ; R, rostrum ; Ri, ribs ; Sp. N, 

 foramina for spinal nerves ; Sy, symplectic ; WS, vertebral column ; aj, foramen 

 for the vagus nerve ; I-V, branchial arches ; II-V, foramina for the optic and the 

 fifth cranial nerves. (From Parker and Haswell, after Wiedersheim.) 



of being directly connected with the periotic capsule, and 

 obviously takes no part in supporting the jaws. Branchial 

 rays for the support of the operculum are attached to the 

 cerato-hyal, and some of them have their bases fused together. 

 The five branchial arches resemble those of the Dog-Fish, except 

 that they tend to become concentrated beneath the skull. 



The existing Chondrostei, 1 and especially the Sturgeon, are 

 remarkable for the persistence and continuous growth of the chon- 

 drocranium, and the absence of true cartilage bones. Numerous 



1 W. K. Parker, Phil. Trans. 173, 1882, p. 139 ; Bridge, Phil. Trans. 169, 

 1878, p. 683. 



