l'.)()7.] XAITKAI, SriKXCES OF PHI LADKI.I'f 1 1 A . 241 



show their oriuiii to ha\'e 1)C(mi among the more generalized forms of 

 the Lower Chiih()rn(\ 



The Jackson and Montgomery races are unquestionably the descend- 

 ants of the Jackson strain in the Lower Claiborne metropolis and are 

 not derived from the Claiborne forms. 



Summary. 



In this study of Athlda petrosa and its allies from the western 

 Gulf region we find the geological series of the races less complete than 

 in the Eocene beds east of the Mississippi River. The Texas Lignitic 

 yields little material, but that little is quite similar to the senile forms 

 of the later Lignitic of Alabama. Texas, however, gives us no such 

 increasingly abnormal series as we find in the Alabama Lignitic. 

 These abnormal forms are considered as derived (1) by migration aritl 

 (2) by senile evolution from a theoretical metropolis situated farther 

 seaward in a more typically marine environment than obtained in the 

 waters of the Lignitic Cnilf. 



With the end of the Lignitic this metropolis is believed to have 

 encroached northward. The Athleta petrosa races of the Texas and 

 Louisiana Lower Claiborne represent this metropolitan assemblage. 

 Among these Lower Claiborne forms we find foreshadowed the Jack- 

 son, IMontgomery and Claiborne races of later time. Li addition 

 Athleta claiji has in this Lower Claiborne diverged from its parent 

 stock, and illustrates one stage of that evolution which finally results 

 in the Claiborne Athleta sayana. The Claiborne forms of Alabama 

 indicate an adjustment to rather local conditions, while the succeeding 

 Jackson is a widespread limestone which bears a clear water facies 

 of the old metropolitan assemblage. 



In dealing with the e\-olution of shell features in fossil gastropods it 

 is necessary to select a group which exhibits change, but at the same 

 time is certainly of one genetic stock. The forms considered in this 

 paper fulfill both of these conditions. No one can doul)t the common 

 origin of the races and species in question, while the morphologic 

 changes are evident. 



It was shown in another paper that tlie Lignitic forms diverged from 

 the main stock by an exaggeration and not by an acceleration of the 

 marks of senility. Their peculiar evolution was attributed to uncon- 

 genial conditions. It was a violent reaction to an unfavorable environ'- 

 ment. The morphologic modifications of the post-Lignitic forms are 

 perhaps still more interesting as casting some light on the laws of 

 gastropodjevolution. Taking the normal species and races of Lower 



