210 ON THE STRUCTURE OF THE SKULL. 



cartilage, to the posterior part of the outer surface of which is 

 applied the bone (F, Fig. 84) with which the opercular bone 

 (Op) is moveably united by ligament. 



The bone (F) has, like most of the bones of the Lepidosiren, 

 a green colour. Through the greater part of its length it is so 

 easily separated from the cartilage that it is clearly a membrane 

 bone. Towards the condyle, however, it adheres firmly to, 

 though, on the application of a certain force, it springs away 

 from, a nodule of whitish bone, which lies in the very substance 

 of the articular end of the cartilage, and repeats its pulley-like 

 form. I suspect that this nodule, which represents the os 

 quadratum, is primitively distinct from the bone (F). The 

 latter, under these circumstances, would have much analogy 

 with the pre-operculum of osseous fishes, and Op would corre- 

 spond with the sub- or inter-operculum. 



All other fishes, comprising such Ganoidei as have not been 

 already mentioned, and the Teleostei, have, so far as is at present 

 known, the palato-quadrate arch primitively distinct from the 

 hyomandibular suspensor ; the latter is, primitively, moveable 

 upon the skull ; and, in the walls of the cranium, the pro-otic 

 bones, at least, are ossified as well as the ex-occipitals ; that is to 

 say, they are constructed essentially upon the plan of the Pike. 

 The modifications they exhibit in detail are almost infinite, but 

 a few of the most important may be enumerated : — 



1. The cartilaginous cranium persists throughout life in such 

 fishes as the Pike and the Salmon ; in very many, as the Perch 

 and the Carp, it disappears almost entirely. 



2. In most fishes the basis cranii is compressed from side to 

 side in the orbital region, and vertically enlarged, so as to form 

 an inter-orbital septum, which, as it were, encroaches upon the 

 cranial cavity and narrows it anteriorly. But in others — such 

 as the Cyprinoids and the Siluroids — no inter-orbital septum is 

 developed, the basis cranii remaining flat, and the cranial cavity 

 of nearly equal size throughout. 



3. The last-mentioned fishes have the cranial walls com- 

 pletely occupied by bone, distinct ossifications representing the 

 alisphenoids and orbito-sjmenoids. 



