242 PROCEEDINGS OF THE ACADEMY OF [^lay, 



Claiborne, Claiborne and Jackson time, we find that on the whole there 

 is much more variation in the smooth stage than an}'T\'here else in the 

 shell. Among the Lower Claiborne races of Athlcta petrosa the only 

 changes worthy of notice are in the length of the smooth stage, while ^4. 

 clayi has departed from .-1. petrosa only by a modification of its apical 

 features. The later shell characters of .4. clayi are almost identical 

 with those of A. petrosa. 



In the Claiborne Sands we have the Claiborne race of Athleta petrosa 

 and A. sayana. The writer suggests that in the first case (.4. petrosa) 

 the local conditions produced a modification in the later stages of a 

 form with a primitive apex — in other words a member of the A. petrosa 

 main stock. In the second case (.4. sayana) it is believed that these 

 similar external conditions acted on a type such as .4. clayi, and that 

 this form with its already' modified apex reacted more readily, so that 

 the characters of its later shell stages departed to a considerable 

 extent from those of .4. petrosa. 



In the more widely extended conditions of Jackson age the apices 

 are not greatly changed in form, but the smooth stage is simply shorter. 

 In other words the curved rib stage and the beginning of the cancel- 

 lated stage are more accelerated. 



From the data furnished by Athleta clayi and .4. sayana it appears 

 that in the normal races of Athleta petrosa and its normal allies marked 

 modifications of the later shell stages are preceded liy marked modifica- 

 tion in the apex. In addition we can say for this restricted normal 

 group at least that the apex is not only a variable feature, but the most 

 variable feature which the shells furnish." 



1^ Tlie author is thoroughly convinced that the features of the apex must be 

 used in classification wth great caution. For this group at least Grabau's 

 generalization does not hold. He says: "The smaller the taxonomic group tlie 

 more nearly identical must be the earlier stages, since in a small group the mem- 

 bers have not become widelv separated from their common ancestor." (.4;/;. 

 Nat., Vol. 36, p. 922.) 



