60 CEREBRAL DEVELOPMENl 



save that it broadens on its upper edges, where in mammals 

 the grinders are placed, so as to furnish field enough for an- 

 gular patches of teeth, which correspond with the angular 

 patches in the palate, it might be regarded, found detached, 

 as at least a reptilian, if not mammalian, bone. The dispo- 

 sition of the palatal teeth of the Dipterus will scarce fail to 

 remind the mechanist of the style of grooving resorted to in 

 the formation of mill-stones for the grinding of flour ; nor is 

 it wholly improbable that, in correspondence with the rota- 

 tory motion of the stones to which the grooving is specially 

 adapted, jaws so hinged may have possessed some such power 

 of lateral motion as that exemplified by the human subject 

 in the use of the molar teeth. 



The protection afibrded by the osseous covering of both the 

 upper and under surface of the cranium of this ichthyolite 

 has resulted, in several instances, in the preservation, though 

 always in a greatly compressed state, of the cranium itself, 

 and the consequent exhibition of two very important cranial 

 cavities, the brain-pan proper, and the passage through which 

 the spinal cord passed into the brain. In the sturgeon the 

 brain occupies nearly the middle of the head ; and there is a 

 considerable part of the occipital region traversed by the spine 

 in a curved channel, which, seen in profile, appears wide at 

 the nape, but considerably narrower where it enters the brain- 

 pain, and altogether very much resembling the interior of a 

 miniature hunting-horn. And such exactly was the arrange- 

 ment of the greater cavities in the head of the Dipterus. The 

 portion of the cranium which was overlaid by what may be 

 regarded as the occipital plate was traversed by a cavity shaped 

 like a Lilliputian bugle-horn ; while the hollow in which the 

 brain was lodged lay under the two parietal plates, and the 

 little elliptical plate in the centre. The accompanying print 

 (fig. 23), though of but slight show, maybe regarded by the 

 reader with some little interest, as a not inadequate repre- 



