288 FISHES 



belongs to the mastiodean is attached the second, and sometimes the 

 only branch of the super-scapular bone, whose third branch, when it 

 exists, is attached more deeply. Under this third ridge, in a fossa 

 scooped under the mastoidean and anterior frontal, the palatine and 

 temporal apparatus are articulated posteriorly, by means of the bone 

 which I call the temporal; it is also from this ridge, that the ridge 

 which goes to form the post-orbitar apophysis of the posterior frontal, 

 is commonly detached. 



The existence, or absence of these prolongations, and their greater 

 or less extent have considerable influence over the particular form of 

 each cranium, and even on that of the whole body of the fish ; thus 

 fishes whose bodies are compressed, and whose backs are raised much 

 above the head, present the middle ridge much elevated also, and the 

 laterals in proportion ; on the other hand, in fishes whose heads are 

 depressed, and bodies round, these ridges are effaced, or are reduced 

 to spinous elevations which are found only at the occiput and from 

 before backwards. When the cranium is both large and flat, the ex- 

 ternal ridges usually form its lateral edges. The arches more or 

 less large, more or less concave, which are sometimes found on the 

 sides of the cranium, as in the cyprius and certain silures, may be 

 classed amongst the most remarkable conformations ; yet their forma- 

 tion depends simply on the greater degree of prominence of some of 

 the parts we have enumerated, or on their union with each other by 

 one or two more sutures, as we shall have occasion to observe in the 

 sequel. It may be therefore affirmed in general, that in bony fishes, 

 whatever be the variations in the general form of their cranium, its 

 composition is nevertheless almost constant, and that the exceptions 

 to this rule, although pretty certain, are by no means numerous. 



Fossa? of the Cranium. 



The superior arch of the great cerebral cavity is formed by the 

 posterior part of the frontals, the parietals, the interparietal, and the 

 external occipital. The posterior frontals and the mastoidean form 

 part of its lateral walls. The orbitary wings are at each side of its 

 anterior wall. The floor is formed by the superior branches of the 

 anterior sphenoid, and by the great wing ; lastly, this cavity termi- 

 minates posteriorly in a canal, which is entirely surrounded by the 

 lateral occipital. This canal, strictly speaking, forms the posterior 

 fossa. The anterior fossa is most commonly altogether membranous ; 

 its place in the skeleton is marked by a large hole, limited laterally by 

 the orbitary wings, above by the frontals, and below by the bifurca- 

 tion of the anterior sphenoid. There are however some kinds, such 

 as the cyprins and silures, in which, as has been already said, the or- 

 bitary wings, and a very large anterior sphenoid unite to furnish, in all 

 its parts, the anterior fossa with bony walls, excepting the orifices 

 necessary for the transmission of vessels and nerves. 



The middle fossa is limited in front by a transverse crest of the 

 orbitary wing, and behind by another, which exists on the internal 

 surface of the great wing, and of the posterior frontal. These two 

 crests are joined behind. At the bottom of this fossa, behind the 

 bifurcated part of the anterior sphenoid, and sometimes, as in the 



