236 



PROCEEDINGS OF THE ACADEMY OF 



[May, 



Athleta petrosa by its large and swollen apex; by its smooth stage 

 which occupies little more than one whorl ; by its more or less suppressed 

 curved rib stage, and by the earlier decline of the spirals immediately 

 below the shoulder. The following table will serve to compare Athleta 

 cJayi with its associate, the St. Maurice race of Athleta petrosa, and also 

 with a probable descendant, Athleta sayana, of Claiborne, Alabama. 



A. petrosa. 



St. Maurice, La. 



A. clayi. 



St. Maurice, La. 



A. Kayana}- 

 Claiborne, Ala. 



Table III. — Showing the greater amount of acceleration in .4. dayi and A. 

 sayana. The less accelerated A. petrosa is introduced for comparison. 



In Table I the compared races of Athleta petrosa are all from one 

 geological formation, the Lower Claiborne. As stated before it is 

 not possible, with our present knowledge of the stratigraphy, to arrange 

 them in the order of their geological succession. This is most unfor- 

 tunate, for it can be seen at a glance that the races differ to a consider- 

 able extent in the acceleration of their shell features. These races 

 may and undoubtedly largely do represent certain episodes in the time 

 range of the stock, but we cannot say which ones are mutations and 

 which ones variations. There appears to be no relation between 

 geographical distribution and degree of acceleration, and even if there 

 were it would mean nothing in view of our ignorance of the stratigraphy. 

 Any attempt at a phylogenetic arrangement of these assemblages, 

 having as its basis the degree of acceleration, is very inadvisable. 

 Such an arrangement would be founded on the assumption that an 

 accelerated form must always and invariably occujDy a higher horizon 

 in the rock series than an unaccelerated form. 



" During 6, 7 and 8 many secondary riblets appear, and these with the spirals 

 produce a secondary cancellation which must not be confused with the primary. 



