340 M. Agassiz on Fossil Fishes. 



noid, the great wings of the sphenoid, and the ethmoid, which 

 form the floor of the cerebral cavity ? It may be said they 

 are apophyses. But the apophyses protect the nervous centres 

 only on the side and above. It may be said that they are the 

 bodies of the vertebrae. But they are formed without the 

 concurrence of the dorsal cord ; they cannot, therefore, be the 

 bodies of the vertebrse. It must, therefore, be allowed that 

 these bones at least do not enter into the vertebral type ; that 

 they are in some measure peculiar. And if this be the case 

 with them, why may not the other protective plates be equally 

 independent of the vertebral type ; the more so because the 

 relation of the frontals and parietals vary so much that it would 

 be almost impossible to assign to them a constant place." 



Microscopic studies had also to furnish their contingent to 

 M. Agassiz's work, since the researches of Mr Owen on the 

 structure of the hard parts of animal bodies, and especially the 

 teeth, have demonstrated that a perfect regularity and a won- 

 derful uniformity exist in the arrangement of the smallest 

 fibres of these organs. The knowledge of these details is par- 

 ticularly valuable for the study of the fossil Placo'ids, of which 

 we possess only teeth and fin rays, the other parts of the skele- 

 ton not being fitted for preservation in a fossil state on ac- 

 count of their soft nature. Even in the existing fauna, there 

 is a group of Sharks, whose teeth are so like each other in ex- 

 ternal form, that it is almost impossible to distinguish them ; 

 for example, the teeth of the Lamnse and those of the true 

 Sharks (Carcharias), or the teeth of the true Sharks and the 

 Carcharodons. But examine their internal structure, and you 

 will find remarkable difi'erences. The same thing applies to 

 the rays of the fins, in so much that hereafter it will be suffi- 

 cient to cut a slice from a tooth or a ray, and to examine it 

 with the microscope, to ascertain correctly to what animal it 

 has belonged. We may, in like manner, determine by means 

 of this ingenious proceeding, even the smallest fragments, pro- 

 vided they are capable of being cut into fine slices. We con- 

 gratulate M. Agassiz on having devoted a certain number of 

 plates to the study of these details, which appears to us destined 

 continually to acquire more importance in palaeontology. The 

 same affinities, the same transitions which take place from one 



