2^0 FISHES 



always distinctive of the group. As a rule, to which, nevertheless, 

 there are notable exceptions, there is little of the primary carti- 

 laginous cranium in the adult, nearly the whole of it having become 

 absorbed or converted into cartilage-bones. A supraoccipital is 

 invariably present, and usually a mesethmoid and a basisphenoid. 

 An additional bone is added to the periotic series, viz. a pterotic. 

 Supra-temporal bones and jugular plates are always absent, and 

 it may be doubted if mento-Meckelian bones and dentigerous 

 splenials are ever developed in the lower jaw. Within the 

 group itself the skull exhibits many notable modifications, of 

 which only a few can here be mentioned. The shape, size, 

 and character of the mouth and jaws, the extent to which they 

 can be protruded and retracted, and the nature of the denti- 

 tion, are the source of many characteristic modifications in the 

 structure and appearance of the fore-part of the skull, and these 

 again largely depend upon differences of habit and food. A 

 protrusible mouth, or a mouth which is projected forwards, is 

 usually associated with a suspensorium (hyomandibular) of con- 

 siderable length, and so greatly inclined forwards as to make a 

 more or less acute angle with the forepart of the cranium. 



The presence or absence of an inter-orbital septum is also a 

 feature in which considerable variation occurs. In some Teleosts 

 there is no septum, and the cranial cavity is prolonged forwards 

 between the orbits, where its lateral walls are formed by well- 

 developed, paired ali- and orbito-sphenoid bones, as, for example, in 

 the Carp and other Cyprinidae. In others the fusion of the cranial 

 walls is accompanied by the median union of the orbito-sphenoids, 

 so that a partly bony and partly cartilaginous inter-orbital septum 

 is found, and the cranial cavity becomes largely obliterated in 

 this region, as in the Salmon ; or the orbito-sphenoids may be 

 non-existent, the cartilage may undergo absorption, and the inter- 

 orbital septum may become reduced to a vertical fibrous sheath 

 extending between the frontals above and the parasphenoid below, 

 as is the case in the Cod (Gadus). 



An interesting modification of certain of the bones of the 

 primary and secondary upper jaw occurs in the Siluridae. In 

 these Fishes the maxillae are very small and edentulous, and 

 serve no other purpose than forming basal supports for the 

 maxillary barbels, while the rod-like palatine bone, losing its 

 connexion with the pterygoid portion of the primitive upper jaw. 



