70 



PROCEEDINGS OF THE ACADEMY OF 



[March, 



characters from every other American species of Volutilithes. Just 

 when it diverged from the V. petrosus main stock the writer is not 

 prepared to say. This race has, properly speaking, no senile stage. 



In order to show more clearly some of the important correlations 

 between the ontogeny and phylogeny, the following tables are intro- 

 duced. The figures stand for the number of the whorl, while the letters 

 are used for the different ontogenetic stages as follows : 



A — Smooth Stage. 



B — Curved Rib Stage. 



C — Cancellated Stage. 



D — Spiny Stage. 



E — Senile Stage. 



I — Individual Variation. 



This last may appear anjnA^here, but it usually follows stage C. B 

 is exaggerated in all the tables. 



V. limopsis. 

 Matthew's Ldg. 



V. petrosus. 

 Gregg's Lclg. Race. 



V. petrosals. 

 Jackson Race. 



E. is very slight. 



E. is never extreme 



Table I. — Showing slow, even evolution in the main stock, accompanied by 

 acceleration. 



V. limopsis. 

 Matthew's Ldg. 



V. petrosus. 

 Gregg's Lag. Race. 



V. petrosus. 

 Bell's Ldg. Race. 



V. petrosus. 

 Wood's Bluff' Race. 



E. is very slight. 

 E. is slight. 

 E. is extreme. 



Table. II. — Showing senility in an offshoot. The senility increases, not by an 

 acceleration, but by an exaggeration of senile features. 



