WOOD, PHYLOGENY OF CERTAIN CERITHIIDM 65 



Horizon and localities : Calcaire grossier. La Frileuse and many other 

 localities in the Paris Basin. 



No. 20157, Columbia University collection. 



Eemarks: The highest development of ornamentation on this shell 

 advances but little beyond the stage with two simple spirals represented 

 by the second to the fourth volutions of P. lamarcM, and the develop- 

 ment of the ribs is so feeble as to correspond with only the earlier part 

 of the fifth volution of that species. P. lapidum is doubtless developed 

 from the same ancestor as P. lamarcki, but since in the adult it becomes 

 more smooth, instead of developing a higher degree of ornamentation in 

 the direction of P. lamarcki, it probably represents a lateral branch from 

 the main Potamides line of evolution. 



4. Potamidopsis 

 Genus Potamidopsis Munier-Chalmas 



1900. Potaiiiidopfiis Munier-Chalmas, Congr§s g§ol. Paris, V, Chideville. Liste 



geuerale, p. 375. 

 1906. Potamidopsis Cossmann, Essais de Paleoconch. Comp., VII, 109. 



Genotype Cerithium tricarinatum Lamarck. 



M. Cossmann distinguishes this genus from true Cerithium as follows : 



Enfin Potamidopsis se distingue des vrais Cerithinw par son canal court, 

 par son labre non replie en travers du canal, et aussi par ses tours imbriqu^s. 

 La separation — qu'a proposee Munier-Chalmas, dans de simples listes de fos- 

 siles publiees a I'occasion du Congres de 1900, sans aucene diagnose — est done 

 a retenir. 



The earliest stages in the development of the genotype show that this 

 shell is derived from the same stock as the true Cerithium, but it diverges 

 from the main line of evolution so strongly and at such an early stage 

 that it deserves to rank as a distinct genus. M. Cossmann describes 

 Potamidopsis as a sub-genus of Potamides, but the group is more closely 

 related to Cerithium than to Potamides; hence it should not rank as a 

 sub-genus of the latter. It is also too distantly related to Cerithium to 

 constitute a sub-genus of that group, and hence it is here ranked as an 

 independent genus. To the distinguishing characters enumerated by M. 

 Cossmann may be added the numerous volutions producing a very high 

 spire ; the close embracing of the whorls ; the general outline of the volu- 

 tions, not convex, but conforming to the slope of the spire; the high 

 development of nodes and the absence of ribs on all except the nepionic 

 volutions of the shell. The aperture also varies in its angular outline — 



