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675 



TERTIARY FAUNA OF FLORIDA 



those who are disposed to make a genus of every well-defined species. In 

 allowing it even a subgeneric place I feel that I am giving it more than its 

 just rank, in view of its very feeble distinctive characters. 



Origin of the Mutations of' Ostrca. 



The oysters are a proverbially difficult group, owing partly to their 

 adherent situs and partly to the fact that they have not hitherto been studied 

 with regard to the direct influence of the environment on individual specimens. 

 That this is very great I have convinced myself from a prolonged study of a 

 multitude of specimens of 0. virginica of which the provenance was known, 

 and of many hundred specimens of our Tertiary species, which usually show 

 from the character of the scar of attachment something of the circumstances 

 in which they grew. The conclusions to which I have been led by this study 

 may be regarded as in part provisional, but in the main highly probable, and 

 as furnishing a first contribution to tlie sort of study which is essential if 

 we would understand the processes of nature through which these animals 

 acquire their most conspicuous external characters. They may be regarded 

 as especially applicable to the Crassostrea group. 



Leaving out of account the nepionic characters, the characteristics of the 

 adult shell may be summarized and derived as follows : The most permanent 

 characters of the shell, and the best, if not infallible, guide to specific recog- 

 nition among the puzzling mutations a large series presents, are the form of 

 the hinge-margin, the minute sculpture of the superficial layer of the shell 

 (often denuded in otherwise perfect fossils), and the sculpture of the valve- 

 margins near the hinge and on each side of it. While not invariable in all 

 specimens, these characters, taken together, will usually enable one to refer 

 the individual to its proper place. 



The characteristics due to situs may be partially summarized as follows : 

 When a specimen grows in still water it tends to assume a more rounded or 

 broader form, like a solitary tree compared with its relatives in a crowded 

 grove. When it grows in a tideway or strong current the valves become 

 narrow and elongated, usually also quite straight. Specimens which have 

 been removed from one situs to the other will immediately alter their mode 

 of growth, so that these facts may be taken as established. When specimens 

 are crowded together on a reef, the elongated form is necessitated by the 

 struggle for existence, but, instead of the shells being straight, they will be 

 irregular, and more or less compressed laterally. When the reef is dry at 



