FOSSIL INVERTEBRATED ANIMALS. 485 



their origin. Some have termed the alveolus, orthoceratite^ 

 and have observed in this part of the structure an evident 

 analogy with that of the ammonites. But we must not, 

 however^ conclude that all that has been called orthoceratites 

 is referable to the alveoli in question. The case of belem- 

 nites has been successively taken for a stem of echinus, or for 

 the tooth or dorsal spine of a fish. It is not so long since M, 

 de Luc advanced that it was the tooth of a soft fish. 



The belemnites are known only in the state of petrifaction. 

 They are usually of a calcareous nature ; and it is very common 

 to find them encrusted in marble with ammonites. Those of 

 Meudon, near Paris, are semitransparent, and in the state of 

 yellowish calcareous alabaster : — their size varies considerably. 

 Some of large dime .sions are found in Sweden and Norway : 

 the ancients discovered them on Mount Ida. 



The living analogues of the belemnites are utterly unknown, 

 and the genus appears to have been destroyed ; or, if it do exist, 

 it is only in the profound abysses of the main ocean. 



Passing over an immense number of the genera of fossil 

 shells, on which nothing general can be advanced of interest, 

 we come to the Terebratulites, or fossil terebratula. This 

 genus, like that of the ammonites, in which the species are so 

 numerous that each locality would seem to furnish some pecu- 

 liar to itself, presents exceeding difficulties in the way of determi- 

 nation ; for the forms of the shell, when the animal is young, 

 differ from those which it exhibits when it has acquired its full 

 dimensions. There are few species of terebratulae in which the 

 under edge is not terminated by folds more or less great, or 

 more or less numerous ; but if we examine one of the shells 

 which has these folds, we shall find that they do not yet make 

 their appearance, while the shell is only half of its proper 

 length. To prevent the mistaking of species for what, in fact, 

 are only the differences of age, it would be right to consider, 

 as species, those shells only which appear to have acquired 

 their full size ; which may be judged of, either from their thick- 



