WOOD, PHYLOGEXY OF CERTAIN CERITHIID^ 59 



The aperture of this species is like that of V. paralhlum, except that 

 the canal is somewhat shorter. 



Horizon and localities : Sables Moyens. Le Guepelle and many localities 

 in tlie Paris Basin. 



No. 20160, Columbia University collection. 



Eemarks: T'. houei is referred by M. Cossmann to the genus Batilla- 

 ria, but a comparison with the type of that genus, B. zonule, shows a wide 

 difference in development, the latter species resembling Cerithium in its 

 early stages. As will be seen by comparing fig. 6, plate vii, with figs. 

 2, 3, 4, plate iii, V. houei differs from Cerithium from the protoconch 

 stage onward, and this difference entitles it to rank as a distinct genus, as 

 noted above. 



3. Potamides 



Genus Potamides Brongniart 



1810. Potamida Beongniart, Ann. du Mus. Nat. d'hist. naturelle, XV, 468. 

 1822. Potamides Beongniaet, in Cuvier's Recherches sur les ossemens fossiles, 



II. 

 1906. Potamides Cossmann, Essais de Paleoconch. Comp., VII, 103. 



Genotype Potamides lamarcki Brongniaet. 



The genus Potamides is distinguished from Cerithium mainly in the 

 slight development of the siphonal canal. In the type of the genus P. 

 lamarcl'i this canal is short, widely open, and with its anterior margin 

 slightly reflexed. This type of canal persists with little change from the 

 middle Eocenic to recent time. 



Tlie early stages of Potamides, as pointed out in connection with the 

 description of the genotype, are closely similar to those of typical Ceri- 

 thium, but after having developed the Cerithium-like outline of the volu- 

 tion with two equal spirals crossed by ribs, species of the genus continue 

 to emphasize the formation of nodes as their most characteristic surface 

 feature, instead of accenting the spirals, as in Cerithium'. In this paper 

 only those species are included in the genus Potamides which have not 

 only an aperture similar to the genotype, but also have young stages in- 

 dicating a similar path of development. 



Cerithium and Potamides are without doubt closely related genera, 

 and Potamides is the more primitive in structure. The aperture of the 

 young Potamides is almost destitute of canal, while the aperture of the 

 young Cerithium is like that of Potamides. and it is reasonable to suppose 

 that the order of evolution has been from forms without canal to those 



