56 FISHES. 



median line ; it constitutes also tlie floor of the orbit. The 

 2)terygoid (24) (or os transversum) starts from the quadrate, and 

 is joined by suture to the palatine, which is toothed, and 

 Teaches to the vomer and anterior frontal. 



In the occipital region there are distinguished the hasi- 

 occipital (5), readily recognised by the conical excavation corre- 

 sponding and similar to that of the atlas, with which it is 

 articulated through the intervention of a capsule filled with a 

 gelatinous substance (the remains of the notochord) ; the cx- 

 occipitah (10), articulated, one on each side, to the basioccipital, 

 and expanding on the upper surface of that bone, so as to 

 meet and support the spinal column ; a superficial thin 

 lamella (13), suturally connected with the exoccipitals, not 

 constant in fishes, and erroneously believed by Cuvier to be the 

 petrosal (os petrosum) of higher animals ; further, the pa7vcci- 

 pitals (9), which are wedged in between the exoccipitals and 

 supraoccijntal. This last bone (8) forms the key of the arch 

 over the occipital foramen, and raises a strong high crest from 

 the whole length of its mesial line ; a transverse supraocci- 

 pital ridge, coming from each side of the base of this spine 

 runs outwards laterally to the external angles of the bone. 

 The supraoccipital separates the parietals, and forms a suture 

 with the frontals. 



In front of the basioccipital the base of the skull is formed 

 by the hasisphcnoid {parasphcnoid of Huxley) (6). This very 

 long and narrow bone extends from the basioccipital beyond 

 the brain-capsule to Ijetween the orbits, where it forms the 

 support of the fibro-membranous interorbital septum. Ante- 

 riorly it is connate with another long hammer-shaped bone (16), 

 the vomer, the head of which marks the anterior end of the 

 palate, and is beset with teeth. The alisphenoids (11) are short 

 broad bones, rising from the basisphenoid ; their j)osterior 

 margins are suturally connected with the anterior of the basi- 

 and ex-occipitals. 



