572 PROCEEDINGS OF THE ACADEMY OF fOct., 



these forms just enumerated exhibit actual morphologic gradations. 

 Any one feature is just a little stronger or more accelerated or perhaps 

 just a little weaker in passing from F. coronatum to the Pliocene 

 F. pyrum — in other words, the sequence is morphologic. 



That the actual specimens from which these tabulated results 

 were obtained could constitute part of a phylogenetic sequence is, 

 of course, inadmissible. The specimens of F. coronatum and F. 

 rugosum were contemporaries in the Miocene sea and the two lots 

 of F. pyrum have their morphologic sequence the reverse of their 

 geologic order. Again, F. canaliculatum and F. pyrum are contem- 

 poraries in the Recent seas and were probably even so in the Pliocene. 



To interpret such data in terms of the phylogeny, one must decide 

 between two quite diverse courses. In the first the worker may 

 accept the geologic sequence as indicating the phylogeny and modify 

 his ideas about organic evolution. The other course hes in choosing 

 the morphologic sequence as portraying the phylogeny and intro- 

 ducing hypothetical species into the final scheme. In this latter 

 case forms which are morphologically discordant but geologically 

 intermediate must be regarded as intercalated migrants which have 

 attained a high degree of specialization. 



As an illustration of the difficulties presented by the first method 

 of interpretation, attention is called to the Recent and Phocene 

 examples of F. pyrum. Those from the Pliocene are according to 

 the morphology at the very limit of the sequence. If the Recent 

 Florida individuals of this species are derived from the Caloosahatchie 

 form, then the canal must have gone through a process the reverse 

 of acceleration, and the disappearance or non-appearance of a final 

 rounded stage must be accounted for in what is probably a majority 

 of Recent examples. When the second method of interpretation is 

 applied to these two assemblages, it is found necessary to derive both 

 Recent and Pliocene examples from a hypothetical F. pyrum. ancestor 

 less specialized than the Pliocene form and not more specialized than 

 the Recent form here tabulated. On this basis the Caloosahatchie 

 F. pyrum is a terminal offshot which is extinct and not the ancestor 

 of the less specialized Recent forms which are regarded as post- 

 pliocene invaders from some other region. 



At first glance it would appear that the method of interpreting 

 by geologic position alone presents fewer difficulties. The study of 

 these closely related species in this as well as in other groups of 

 gastropods has, however, led the author to favor not the first, but 

 the second method of interpreting. The r(>asons for choosing such 



