2l6 FISHES 



CHAP. 



present in the Salmon. There is usually a median ossification of the 

 ethmoid cartilage, the mesetJimoid. 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 corona7-t/ bones on the inner side, and a sv,pra-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. Bhizodopsis) fringe the outer margin of the jaw 

 beneath the true dentary (Fig. 274, A). A system oi 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 {sup)ra-tempoTals) 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 differing in the two subdivisions. In the Lamprey ^ (Fig. 

 124) the paired parachordals add trabeculae together form a 

 trough -like chondrocranium, which has only a fibrous roof, 



1 M'Murrich, Proe. Canadian Jnst. (S.S.) ii. Toronto, 1884, p. 278; Cole, 

 Trans. Linn. Soc. vii. Ft. v. 1898, p. 131. 



2 W. K. Parker, Phil. Trans. 174, Pt. ii. 1883, p. 411 ; Huxley, Joum. Anat. 

 and Phys. x. 1876, p. 412 ; Howes, Trans. Biol. Soc. Liverpool, vi. 1891, p. 122. 



