240 



PROCEEDINGS OF THE ACADEMY OF 



[May, 



this case the first change was a modification of the apical characters 

 {A. clayi), and the second change was modification in the featm'es of the 

 spiny stage (.4. say ana). The following table probably comes reason- 

 ably close to the phylogeny of these forms. 



It will be seen that in the above scheme the Claiborne race of Athleta 

 petrosa is placed in a position which indicates a slight divergence from 

 the main stock. This change in its characters is attributed to the 

 local nature of the Claiborne Sands environment.** The Claiborne 

 race of A. petrosa is the type of the species, and this is just one example 

 of w^hat is frecjuently encountered in dealing with large series of races 

 or individuals, namely, that the specific type may not be the most 

 typical expression of the actual specific group. ^'^ 



The races of Athleta petrosa which are displayed in the Lower Clai- 

 borne deposits of Texas and Louisiana appear to constitute a veritable 

 metropolis. In this Lower Claiborne metropolitan assemblage was 

 evolved Athleta clayi. The Claiborne race of A. petrosa and A. soyana 

 of the Alabama Claiborne Sands represent local conditions, and clearly 



" See A. C. Veatcli, Geol. Surv. La., 1902, p. 140. 



'* Prof. Henry S. Williams has observed that "species are often described for 

 the extreme form rather than the central stock." He also lays stress on "the 

 necessity of distinguisliing the real specific group from the species as understood 

 in literature." 



