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 



«r 



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

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

 articular ; h, epi-branchial ; c, cerato-branchial ; C, notochord ; Cop, basi-branchials ; 

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

 cerato-hyal ; J/i, inter-hyal ; Md, lower jaw ; JVa, 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 ; x, foramen 

 for tiie 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,-' 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. 



