Fig. 129.—Lateral view of skull of Chimaera monstrosa. a.s.c, Position of anterior semicircular canal; c.hy, cerato-hyal; e.hy, epi-hyal; fr.cl, frontal clasper; h.s.c, position of horizontal semicircular canal; i.o.s, inter-orbital septum; lb.1, lb.2, lb.3, labial cartilages; Mck.C, mandible; Nv.2, optic foramen; Nv.10, vagus foramen; olf.cp, olfactory capsule; op.r, opercular rays; pal.qu, palato-quadrate; ph.hy, pharyngo-hyal, or hyomandibular; p.s.c, position of posterior semicircular canal; qu, quadrate region; r, rostrum. (From Parker and Haswell, after Hubrecht.)
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 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.
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.
Fig. 131.—Lateral view of the primary and secondary upper and lower jaws of Polyodon. b.br′, First basi-branchial; ch, cerato-hyal; d, dentary; hy.h, hypo-hyal; hy.m, hyomandibular; i.hy, inter-hyal; i.op, inter-operculum; lgs, ligaments connecting the palato-quadrate cartilage with the hyomandibular; mk.c, Meckel's cartilage; mx, maxilla; op, operculum; pa, palatine; pa.q, palato-quadrate; ps.l, pre-spiracular ligament; q, quadrate cartilage; sym, symplectic. (From Bridge.)
Numerous dermal bones invest the dorsal surface of the chondrocranium, and only to a limited extent correspond with the less numerous membrane bones of the Salmon. To these are added a series of circum-orbital bones and a large parasphenoid. Undoubtedly the most striking feature in these Fishes is the primitive character of the upper jaw. In Polyodon (Fig. 131) the palato-quadrates are wholly cartilaginous, and, as in the Dog-Fish, they meet in front beneath the basis cranii, where the two are connected by ligament. The secondary upper jaw is but feebly developed, and is represented on each side by a thin splint-like maxilla in relation with the outer surface of each palato-quadrate cartilage, which meets its fellow in front. There are no premaxillae. The lower jaw is also very primitive. Meckel's cartilages are persistent, and except for a mento-Meckelian bone on each side, they are unossified, although membrane bones representing dentary and splenial elements are present. The skull is hyostylic. The hyoid and branchial arches are only partially ossified. Each opercular fold is supported by an operculum and an interoperculum, and both of these retain somewhat the shape of the cartilaginous hyoidean rays which they have replaced. In the Sturgeon (Fig. 130) the upper jaw is greatly modified in relation with the singular mouth of this Fish. The palato-quadrate cartilages meet not only in front, but also along their dorsal margins, and, with the help of the similarly opposed and somewhat fragmentary metapterygoid cartilages, they form a complete concave roof for the protrusible spout-like mouth. Palatine, mesopterygoid, and pterygoid bones invest, and in some measure replace these cartilages. In brief, the skull of the Chondrostei occupies an interesting intermediate position between the purely cartilaginous and mainly bony types. While retaining a well-developed and unossified primary cranium, it has acquired a complete secondary cranium of dermal bones. Equally notable is the condition of the jaws. Unique among the Teleostomi in possessing the typical Elasmobranch union of the palato-quadrate cartilages beneath the basis cranii, the Chondrostei are so far specialised that they have acquired certain of the membrane bones which constitute the secondary jaws of the more typical bony Fishes.
As regards the general structure of the skull and the nature and disposition of its cartilage- and membrane-bones, the remaining living Teleostomi have much in common with the Salmon. In all the skull is hyostylic, and, unlike the Chondrostei, each half of the primitive upper jaw remains distinct from its fellow, and is separately articulated in front with the lateral ethmoid of the same side by its palatine element. The palato-quadrate cartilage is always more or less completely replaced by bones similar to those of the Salmon, and although they often carry teeth, as a rule they do little more than constitute a rigid buttress for the fixation of the quadrate condyle for the lower jaw. The secondary upper jaw is nearly always well developed, and includes a premaxilla as well as a maxilla on each side. There are, however, certain features in each of the minor groups which are either distinctive or highly characteristic.