452 



cojMparative anatomy. 



chord, and wliicli corresponds to tlie anterior cavity of tlie first 

 vertebral centrum. At its sides tlie exoccipitals {01} are attached to 

 it ; these always enclose the greater part of, and sometimes nearly 

 all, the posterior foramen magnum. The supra- occipital {Os) enters 

 into it above ; this is often distinguished by a large vertical process, 

 which resembles the spinous processes of the vertebral column. 



The next segment partly, at any rate, encloses the labyrinth, and 

 the bones which compose it are named in accordance with this rela- 

 tion. The most constant, and therefore the most important, is the 

 petrosal (pro-otic) ; this contains, or, at least, limits posteriorly, the 

 foramen of exit of the trigeminal nerve. It extends to the base of 

 the skull, where it may become united with its fellow of the opposite 

 side, within the cavity of the skull. The epiotic forms a second 

 piece; this is attached above to the exoccipitals, and generally forms 

 a projecting process (Fig. 243). A third one, the opisthotic (inter- 



calare), generally lies to the sides 

 and front of the exoccipital, and 

 varies greatly in character (Figs. 

 243-6). As a rule this piece has 

 no relations to the labyrinth, 

 with which, however, other bones, 

 such as the exoccipital, very 

 frequently become connected. 

 Lastly, an external covering 

 piece of the primordial cranium 

 belongs to this region, and 

 gradually becomes closely united 

 with it. It takes part in the for- 

 mation of the articulation for the 

 hyomandibular, and often develops a process which projects back- 

 wards and outwards. This is the squamosal (Figs. 242, A Sq ; 243, 6). 

 Great differences may be seen to obtain in the development of the 

 bones of the next portion, which depend on the size of the cavity of 

 the skull. When this extends far forwards, the walls of the primor- 

 dial cranium are correspondingly more complete, while, when it is 

 reduced in size, its walls are diminished, and are partly replaced by 

 membranous structures. Thus, in many cases, there is a mem- 

 branous interorbital septum ; or bones, which are well developed in 

 other forms, are present in a rudimentary condition. 



The ossifications of this segment are the alisphenoid (sphenoi- 

 dale laterale posterius) at the sides and behind, and the orbito- 

 sphenoid (sphen. later, anter.) in front. In Amia, as in many Tele- 

 ostei, the latter are separated from one another, while m other forms 

 the pieces of either side unite at the base of the cranial cavity, and 

 at last may fuse into one piece, or may be rudimentary. At the base 

 of this segment there is a basi-sphenoid, which is derived from the 

 cartilage of the primordial cranium ; this is, as a rule, a bone of no 

 great size ; it is connected superiorly Avith the alisphenoid. When 

 the base of the skull is traversed by a canal for the optic muscles. 



Fig. 243. Posterior portion of a cranium 

 of Gadus (side view). 1 Basi-occipital. 

 2 Exoccipital. 3 Supra-occipital. 5 Para- 

 sphenoid. 6 Opisthotic. 6' Squamosal. 

 7 Epiotic. 15 Prootic. 12 Postfrontal. 

 11 Frontal, c Articular surface for the 

 hyomandibular. 



