486 FOSSIL INVERTEBRATED ANIMALS. 



ness, or where the locality presents a good number of indivi- 

 duals of the same size and form. 



The same locality often presents us with one species smooth 

 and another folded ; but a great number of species are not 

 found mingled together. 



The terebratulae shew themselves in the strata anterior to 

 the chalk, in those of that substance where they have almost 

 always preserved their testa, and in the coarse limestone. But 

 it does not appear that they are found in more recent strata 

 than the last; and, indeed, they seem less frequent, according 

 as the strata where they are found are less ancient. Some 

 ancient strata appear to be composed of nothing else. 



M. Lamarck has stated that this genus may be divided into 

 some others ; and, in fact, some which are pierced with a round 

 hole at the summit of the largest valve, appear to have been 

 attached by a tendinous pedicle, like those which are known 

 in the living state. But there are others in which this hole is 

 entirely wanting ; others appear to have had a triangular hole 

 above the beak. 



There are, in the species of the terebratulse, as in those of 

 other genera, individual differences, and others which may arise 

 from locality, either in magnitude or form ; so that, doubtless, 

 varieties have often been taken for species. 



The genus of the Oysters, which, in a fossil state, are termed 

 ostracites, presents a very considerable number of species. 

 They are found in all the shelly strata, from those of the am- 

 monites inclusive to the most recent. They are so common, 

 in certain localities, that many of these, to the extent of many 

 miles, are entirely composed of them ; as, for example, Mount 

 Audona in Piedmont. 



As the testa of oysters does not disappear, when they are 

 found in localities where that of other soluble shells has disap- 

 peared, it frequently happens that we find them alone in strata, 

 where they were accompanied by shells of other genera no 

 longer found there. 



