324 Prof. Agassiz on Fossil Fishes. 



have said, of detached vertebrae, or of isolated crania. The 

 latter, moreover, are generally deprived of the bones of the 

 face ; the jaws, the opercular and branchial apparatus are want- 

 ing, and most frequently only the cranial envelope, properly 

 so called, remains ; and very often, indeed, even this has lost 

 the whole of the anterior portion, the snout formed by the 

 union of the nostrils and of the vomer, so that there is no 

 other point to start from than the cranium, deprived of all its 

 appendages. To determine these fragments, I have followed 

 the same process that nature employed to place these fossils in 

 the state in which we meet them. Ordinary skeletons, such 

 as are contained in the museums of natural history and com- 

 parative anatomy, would not have sufficed for my purpose. I 

 began, therefore, by preparing a certain number of detached 

 bones of different marine fish, and I possess at present a hun- 

 dred detached crania with the other bones separated, — a collec- 

 tion which I am daily increasing. As it is of importance that 

 the different bones of the cranium be not isolated, but that 

 the envelope which they form should preserve its natural form, 

 all these crania have required the greatest care in their pre- 

 paration ; and in this a great difficulty occurred, arising from 

 the manner in which the bones of the cranium are joined in 

 fish. In the other Vertebrata this junction is effected by su- 

 tures; the crenulate and dentate margins correspond, and it is 

 easy to reconstruct a dismembered cranium. In fishes, such 

 is not the case. Most frequently the bones are applied on 

 an internal cartilage form, frequently very thick, sometimes 

 thinner ; and their margins, if indeed they touch, are applied 

 the one on the other by their faces, or separated by broad 

 bands of cartilage. The general form of the cranium is, there- 

 fore, frequently entirely different from what it would be if we 

 were to attempt to reconstruct the cranium with isolated bones, 

 approximating these latter by their margins. In the fish from 

 Sheppey the cartilages have disappeared, the clay has taken 

 their place, but not entirely, so that the crania have the form 

 which the skulls of half-dried recent fish acquire. It is this 

 point of desiccation which I have endeavoured to attain in my 

 crania of recent fish. 



These means of comparison might appear sufficient, did we 



