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 {223}(_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] [Illustration: 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.)]