In the Holocephali (_e.g._ _Chimaera_[197]) the cranium retains its primitively cartilaginous condition, and assumes a somewhat peculiar appearance owing to the lateral compression and vertical growth of its inter-orbital and nasal regions (Fig. 129). There is a complicated series of labial cartilages in relation with the ventrally-placed nostrils and the upper and lower jaws. In the males of _Chimaera_ and _Callorhynchus_, but not in _Harriotta_, a movable cartilage is attached to the cranial roof, and supports the frontal clasper. The skull is typically autostylic. Along {224}the whole length of its dorsal border the palato-quadrate cartilage 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 articular 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 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. [Illustration: FIG. 130.—Side view of the skull of a Sturgeon, with the investing membrane bones removed. _a_, Pharyngo-branchial; _AF_, antorbital or lateral ethmoid cartilage; _AR_, articular; _b_, epi-branchial; _c_, cerato-branchial; _C_, notochord; _Cop_, basi-branchials; _d_, hypo-branchial; _De_, dentary; _GK_, auditory capsule; _Hm_, hyomandibular; _hy_, cerato-hyal; _Ih_, inter-hyal; _Md_, lower jaw; _Na_, nasal capsule; _Ob_, neural arches; _Orb_, Orbit; _PF_, post-orbital process; _PQ_, palato-quadrate; _Ps_, _Ps′_, _Ps″_, parasphenoid; _Psp_, neural spines; _Qu_, quadrate; _R_, rostrum; _Ri_, ribs; _Sp.N_, foramina for spinal nerves; _Sy_, symplectic; _WS_, vertebral column; _x_, 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.)] The existing Chondrostei,[198] and especially the Sturgeon, are remarkable for the persistence and continuous growth of the chondrocranium, and the absence of true cartilage bones.