323 



no stages later than the cancellated stage. There is also very little in- 

 dividual variation. Associated with A. limopsis is the species A. rugatus. 

 In its earlier stages this species very closely resembles A. limopsis, but 

 "differs radically .... from that form with the progress of its 

 ontogeny." In its later whorls it presents evidence, though not extreme, 

 of senility. It has no spiny stage. 



The next species A. petrosa, represents an assemblage of races con- 

 nected by many intergrading forms. These races range upward from the 

 Nanfalia beds to the Jackson beds of the Eocene. Several of them are 

 senile races, and in the adult strikingly different from the ancestral form, 

 A. limopsis. Smith says, however, that the young of all the races "are 

 remarkably uniform and constant. The early whorls indicate clearly that 

 they are all descended from a cancelated ancestor, and bear a strong re- 

 semblance .... to the characters of A. limopsis." Some of the 

 senile races of petrosa are profoundly modified in the adult, as for ex- 

 ample, the Hatchetigbee race, derived from the main stock through the 

 Bell's Landing and Wood"s Bluff races. Yet their derivation from the 

 main stock is shown by intermediate forms, and the young of the terminal 

 races greatly resemble the ancestral form. In the Jackson race, which is 

 the terminal member of the main stock, the last two whorls are spiny, 

 and the last whorl shows some senile characters at its close. "This race 

 shows a regular and even ontogeny." Acceleration has carried the curved 

 rib stage back to the beginning of the third whorl, whereas in the an- 

 cestral A. limopsis this stage begins near the close of the fourth whorl. 



Smith has graphically expressed the main developmental and phylo- 

 geuetic changes in the following diagram : 





1 2 



3 



4 



5 6 ' 7 



8 



9 1 10 



i 



11 



12 



A. limopsis. 



| 



A 



B 







C 













'" '1" ' 





' 











A. petrosa. 





A 





B 



... c 







D 



and 

 I 



D 



and 

 I 





E Slight 













A. petrosa. 



A 





B 





C 









D 



D 

 and 



E 















Upper Eocene. 



In the above diagram the figures across the top stand for the number 

 of the whorl of the shell, and the letters indicate the different ontogenetic 

 stages as follows : 



