of tJie Vertebrate Skeleton. 35 



basis of a shark's skull, it would be quite consistent Avitli the 

 vertebrate plan to have a greater number of superior arches 

 than of median base-bones. 



But in those ordinary osseous fishes in which the several 

 bones can be separated from each other, we find the skull in 

 no transitional state, but already with the elements well de- 

 fined, except at the base of the skull, where the kinetic ossifi- 

 cation persists as a long median bone called the parasphenoid 

 or basitemporal. And in the upper part of the skull, besides 

 the three ordinary arches such as have been described, there 

 come to be introduced three additional, imperfect arches, ana- 

 logous to the intervertebral neural arches of sharks, and which 

 I interpret as potential representatives of those structures. 

 The first pair, in front of the frontal bones, are named the 

 prefrontal bones, one on each side ; the second pair are be- 

 tween the frontal bones and parietal, and are named post- 

 frontal ; the third set are between the occipital and parietal, 

 and are named the interparietal bones : these latter only per- 

 sist in the skulls of the higher Vertebrata. 



It is to be remarked that in fishes the cranial bones overlap 

 each other in the squamous way in which an ordinary zyga- 

 pophysis laps upon its fellow. 



And it appears to me probable that Prof. Owen truly appre- 

 ciated the homology of the bones which roof in the skull when 

 he compared them to the small ossification which often 

 crowns the spinous part of the vertebral neural arch, which is 

 by him named the neural spine, since without that ossification 

 it would be more difficult to see why the lateral bones should 

 not curve upward and roof in the cranium. 



It is also worth considering whether in osseous fishes the 

 potential growth may not have a direction, so to speak, given 

 it by the influence of cerebral form, because it is observed, in 

 skulls of equal size, that in Lopliius^ which has the cerebellum 

 very short and small, the occipital region of the skull only 

 measures 2 inches in length, while in the tunny, which has the 

 cerebellum large, the occipital part of the skull measures 4^ 

 inches in length ; so that, since some fishes (like the eels) have 

 olfactory lobes to the brain almost as large as the cerebrum, 

 it may not be impossible that such a condition in fishes may 

 have had a tendency to promote differentiation like that seen 

 in the separation of nasal bones from the prefrontal in some 

 Clielonians. 



Now, just as in the more osseous fishes the parts of the 

 divided neural arch become blended, and the centrum becomes 

 more solid, so in the higher Vertebrata the prefrontal and post- 

 frontal bones have become lost under the uniformity induced 



3* 



