No. 440.] 



S7T/>//:S OF GASTROPODA. 



Ancestral Species. 



An immediate ancestor of S. pyrus seems to be S. pyrifonnis 

 (Conrad) from the late Miocene of the Natural Well, Duplin 

 Co., North Carolina. 1 In this species the tubercles continue 

 nearly as long as those of S. canaliculars ; they become obso- 



The last portion of the sixth volution is rounded, as in adult 5. 

 Pyrus. 5. pyriformis is therefore a less accelerated type than 

 .V. pyrus, and fulfills all the requirements of the immediate 

 ancestor of the latter. 



The next earlier representative of the series seems to be a 

 form from the Miocene of Faison, N. C, and probably referable 

 to S. canalifcnts (Conrad) Gill. The type of this species is from 

 the early Pliocene (Waccamaw beds of South Carolina, Tuomey 

 and Holmes), and is a more advanced type, not far removed from 

 the recent S. canaliculars. (See Jhtsicon canaliculatum, Tuomey 

 and Holmes, Pliocene Fossil Shells of X. Carolina, pi. 29, fig. 

 2). This species is considered by Dall as the ancestor of S. 

 canaliculars. It fulfills the requirements of such a relation- 

 ship, in that the keel is tuberculated throughout. While 5. 



ferns S. pyrus and \. pyriformis have passed two stages beyond 

 this. Therefore a species of the .9. pyrus type only as far 

 advanced as X caualicnlatns, i. e., one in which the last whorl 

 is keeled but not rounded, might be looked for in the upper 

 Miocene. These conditions seem to be satisfied by species 

 occurring in the late Miocene beds of Faison Mt. Pass, North 

 Carolina. 



Other specimens found in these later Miocene beds of Faison, 

 N. C, show well the manner of formation of the sutural channel 

 in retarded as well as primitive types. In the simplest specimen 



the conch. This becomes wider, more pronounced, and faintly 



