708 LOUIS AGASSIZ. 



and their order of succession in geological 

 times (see " Essay on Classification ") justifies 

 another form of expression of these facts, 

 namely, that in deeper waters we should ex- 

 pect to find representatives of earlier geolog- 

 ical periods. There is in all this nothing 

 which warrants the conclusion that any of 

 the animals now living are lineal descendants 

 of those of earlier ages ; nor does their simi- 

 larity to those of earlier periods justify the 

 statement that the cretaceous formation is still 

 extant. It would be just as true to nature to 

 say that the tertiaries are continued in the 

 tropics, on account of the similarity of the 

 miocene mammalia to those of the torrid zone. 

 We have another case in the Pleurotomaria. 

 It is not long since it has been made known 

 that the genus Pleurotomaria is not altogether 

 extinct, a single specimen having been discov- 

 ered about ten years ago in the West Indies. 

 Even Pictet, in the second edition of his Pale- 

 ontology, still considers Pleurotomaria as ex- 

 tinct, and as belonging to the fossiliferous 

 formations which extend from the Silurian 

 period to the Tertiary. Of the living species 

 found at Marie Galante, nothing is known ex- 

 cept the specific characteristics of the shell. 

 We dredged it in one hundred and twenty 



