1906.] 



NATURAL SCIENCES OF PHILADELPHIA. 



71 



10 11 12 



T''. limopsis. 

 Matthew's Ldg. 



T'. petrosus. 

 Gregg's Ldg. Race. 



V. petrosus. 

 Claiborne Race. 



V. sayana. 

 Claiborne. 



E. is very slight. 



I. is great. 



D. is much modi- 

 fied. I. is great. 



Table III. — Evolution of F. sayana through the Claiborne race of V. 

 petrosus. Shows general acceleration, condensation of B, and reduction 

 in the number of whorls. 



12 3 4 5 (•, 7 8 10 11 12 



V. limopsis. 

 Matthew's Ldg. 



V. rugaius. 

 Matthew's Ldg. 



I. is the transition 

 between C. and E 



Table IV.— Shows evolution of V. rugatus from the primitive ancestral stock 

 by the appearance of senile features in the last whorl or in the last two 

 whorls. 



Range and Distribution. 



Of the forms considered in this paper Volutilitkes limopsis stands out 

 distinctly as fulfilling all the requirements of an ancestor. Among 

 other characters, the long smooth stage and the absence of senile fea- 

 tures show its primitive state. The general likeness of the full-grown 

 shells to the young of many later species, together with its low geo- 

 logical position, justifies us in considering it to be the form from which 

 were descended the races which followed Matthew's Landing time. 



In the same horizon with V. limopsis we find V. rugatus. The young 

 of the latter show many of the features of the adult V. limopsis, while 

 the old individuals are often slightly senile and quite different in ap- 

 pearance from the small specimens. It was undoubtedly derived from 

 V. limopsis, or some closely allied form, and may be considered a short 

 senile offshoot which did not survive the Matthew's Landing. 



