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1003 



TERTIARY FAUNA OF FLORIDA 



the species it is not absolutely constant, though mainly so. Physiologically, 

 as I have already pointed out, it has very little significance. In general the 

 tendency to coalescence increases with the progress of geological time, but 

 even in the Eocene there are species with wholly coalescent scars. In a 

 general way species which would be placed together on other grounds have 

 similar sinus characters, but there are so many exceptions to this that no rule 

 can be said to be established. 



In this group, as in the gastropods, singular as it may appear, the char- 

 acters of the external sculpture seem to be among the most permanent features 

 in an evolutionary series from the Eocene to recent times. All of the groups 

 approach each other closely, through peripheral species, in this as in other 

 features. 



A careful examination of the hinges of a large number of species in- 

 dicates that the lateral laminse are prone to become obsolete in all the forms 

 where they are not actively functional. The right anterior lateral when adja- 

 cent to the cardinals is invariably functional, which may account for its 

 exceptional constancy. In species where a lateral is represented only by an 

 almost microscopic ridge or pustule it becomes difficult to decide on its diag- 

 nostic use. One cannot say the species has no laterals, although they are obso- 

 lete, and it sometimes happens that species, closely allied by other characters, 

 differ in the state of the laminae, so that if these organs were functional the 

 discrepancy is such as ordinarily would be taken to be of sectional or sub- 

 generic value. 



In my diagnoses of groups I have described as carefully as I could the 

 characters of the type species, but it will frequently happen that the forms 

 which it seems necessary to associate with the type do not correspond in all 

 minor details with the diagnosis. I cannot bring myself to think that a named 

 subdivision for each of these fluctuations would correspond to any important 

 series of facts or be of real service to students ; indeed, I have found the 

 multiplication of ill-defined and insufficiently compared subgenera, sections, 

 etc., in much of the later literature a real impediment to study. 



Realizing the difficulties, and that I can hardly hope to surmount all of 

 them to the satisfaction of everybody, I have tried at least to make my state- 

 ments correspond with what I have seen in the specimens, and have not 

 troubled myself overmuch as to whether this agreed with antecedent literature 

 or not; except that, when I found a disagreement, I have taken the pre- 

 caution to review my work at intervals and again compare with the specimens. 

 If any reader feels disposed to criticise the result, my only request is that 



