Fig. 125.—Side view of the skull of Bdellostoma; the gill-apertures and their cartilages have been omitted. A, Auditory capsule; B, B′, B″, the anterior, middle, and posterior segments of the lingual bar; b1, cartilage connecting the hyoid arch with the second branchial arch; br1, br2, first and second branchial arches; c.c, coronal cartilage; Cr, cranium; D, dental plate; dt, median dorsal tooth; Ex.n.c, external part of the naso-pituitary canal; Hp, hypophysial plate; Hy, hyoid arch; N, subnasal cartilage; nc, neural canal; Nt, notochord; OC, olfactory capsule; PL, palatine portion of the palato-quadrate cartilage PQ; S, supra-pharyngeal plate supporting the velum; t, tendon of the retractor mandibuli muscle; t1, t2, t3, tentacular cartilages; t4, cartilage supporting mouth lobe; tr, trabecula; V1, rod connecting S with the inner surface of the hyoid arch of its side; V, outer lateral rod which joins V1; 1, 2, 3, fenestrae. (Modified from Ayers and Jackson.)

While the skull of the Myxinoid Cyclostomes[[194]] is constructed on the same general lines as that of the Lamprey, it is in some respects more primitive. It is also clear that in other features the skull has undergone marked specialisation on lines of its own, and in some points again it seems to deviate less from the more normal Craniate type. Of the more obvious differences, as illustrated by the skull of Bdellostoma (Figs. 125-127), it will be sufficient here to mention the following: (i.) The more primitive condition of the chondrocranium, the roof and side walls of the cranial cavity being entirely membranous. (ii.) The non-development of a suctorial buccal funnel and the presence of oral tentacles, associated with the absence of the complex system of labial cartilages and the substitution of a special tentacular skeleton. (iii.) The special modifications induced by the length and physiological importance of the naso-pituitary canal and by its communication with the pharynx after perforating the pituitary fontanelle in the cranial floor.

Fig. 126.—View of the upper surface of the dental plate of Bdellostoma. t, Tendon of retractor muscle. (From Ayers and Jackson.)

Fig. 127.—Dorsal view of the skull of Bdellostoma. Reference letters as in Fig. 125. (After Ayers and Jackson.)

Under this head may be included the depression of the mesethmoid or hypophysial plate for the support of the naso-pituitary canal, the forward prolongation and median union of the palato-quadrate cartilages of opposite sides beneath the external portion of the canal, apparently for the support of the latter, and the encircling of the canal by supporting annular rings of cartilage. (iv.) The presence of two branchial arches, connected, as in Fishes, with a median basi-branchial segment which forms the middle one of the three divisions of the lingual apparatus. (v.) The reduction of the complicated extra-branchial basket to small isolated cartilages in relation with the gill-apertures and the œsophago-cutaneous duct. (vi.) The extraordinary development of the lingual apparatus, of which it has been remarked that it "dominates the whole body, everything else yields to it." Meckel's cartilages are represented either by the cornual cartilages, as seems most probable, or by the dental plate (Fig. 125, c.c. and D).

Fig. 128.—Lateral view of the skull of Notidanus (Heptanchus) cinereus; mck, Meckel's cartilage, or primitive lower jaw; pal.qu, palato-quadrate cartilage or primitive upper jaw; pt.orb, post-orbital process of the cranium with which the post-orbital process of the palato-quadrate articulates. (From Parker and Haswell, after Gegenbaur.)

In the generality of Elasmobranchs the skull resembles that of the Dog-Fish in essential structure. The more important modifications within the limits of the group relate to differences in the mode of attachment of the primitive upper jaw to the skull, and the number of branchial arches. In most Elasmobranchs the skull is hyostylic, as in Scyllium, but there are two genera which, in different ways, are exceptions to this rule. In Notidanus the hinder part of each palato-quadrate cartilage grows upwards into a strong post-orbital process, which articulates with the suitably modified post-orbital process of the periotic capsule (Fig. 128); hence the primitive upper jaw acquires a direct dorsal connexion with the cranium, and, as the hyoid arch is now relieved from taking any part in its support, the hyomandibular is reduced to the condition of a relatively slender rod of cartilage. By this arrangement both the mandibular and hyoid arches have their own separate and independent connexions with the cranium, and the skull is said to be amphistylic.[[195]] The Port Jackson Shark (Heterodontus) exhibits another and quite different modification. In this Fish the dorsal border of each palato-quadrate cartilage fits into a deep groove along the infero-lateral surface of the cranium, and is firmly attached thereto by ligament. Thus the first step is taken towards that more complete fusion of the two structures which is so characteristic a feature in the more typically autostylic Fishes like the Holocephali and the Dipnoi. Autostylism, whether incipient, as in Heterodontus, or complete, is to be regarded as a secondary modification, which may be independently acquired in widely different groups of Fishes, and is usually associated with the need of a firm and rigid support for an exceptionally massive dentition.[[196]]