216 FISHES CHAP. 
present in the Salmon. There is usually a median ossification of the 
ethmoid cartilage, the mesethmoid. An entopterygoid is sometimes 
added to the palato-pterygo-quadrate series of bones. An ossifica- 
tion of the anterior extremity of each Meckelian cartilage may form 
a mento-Meckelian bone. Certain additional membrane bones are 
sometimes developed in relation with the lower jaw, such as splenial 
and coronary bones on the inner side, and a swpra-angular bone at 
the angle of the jaw, above the angular element. To these there may 
be added the singular series of infra-dentaries, which in some fossil 
Crossopterygii (e.g. Rhizodopsis) fringe the outer margin of the jaw 
beneath the true dentary (Fig. 274, A). A system of jugular plates 
may also form a characteristic armature for the throat between the 
lateral halves of the lower jaw (Fig. 274,C). Besides those already 
mentioned, additional sensory canal bones are present In some 
Fishes. A transverse row of plates (supra-temporals) sometimes 
crosses the occipital region behind the parietals. There are also 
other canal-ossicles which lose their identity by fusing with certain 
cranial or periotic bones. Thus, each of the pterotic and sphenotic 
bones often includes a superficial dermal bone transmitting a section 
of a sensory canal, which has fused with it; and as the frontal 
bone is often similarly perforated, it may be taken that it also 
includes a canal-ossicle; and the same can often be said of the 
articular and dentary bones of the lower jaw.! 
Having now considered the general structure of a primitive 
cartilaginous type of skull, and the nature, disposition, and 
terminology of the various membrane- and cartilage-bones which 
may be added to, or more or less completely replace the former, 
reference will now be made to the more important features in 
the structure of the skull in the Cyclostomata and the Fishes. 
In the Cyclostomata the skull presents a remarkable combina- 
tion of characters, in some of which it is more primitive than in 
any other Craniates, while in others it has evidently attained a 
very high degree of specialisation on lines peculiar to the group, 
but differmg in the two subdivisions. In the Lamprey? (Fig. 
124) the paired parachordals and trabeculae together form a 
trough-like chondrocranium, which has only a fibrous roof, 
1 M‘Murrich, Proc. Canadian Inst. (N.S.) ii. Toronto, 1884, p. 278; Cole, 
Trans. Linn. Soc. vii. Pt. v. 1898, p. 131. 
2 W. K. Parker, Phil. Trans. 174, Pt. ii. 1883, p. 411; Huxley, Journ. Anat. 
and Phys. x. 1876, p. 412; Howes, Trans. Biol. Soc. Liverpool, vi. 1891, p. 122. 
