WOOD, PHYLOGENY OF CERTAIN CERITHIIDJE 69 



Potamidopsis tricarinata mut. eris mut. nov. 



1824. Cerithiiun tricarinaium var. e Deshayes, Desc. des coquilles foss. des 

 environs de Paris, II, 326, pi. 51, figs. 4, 7. 



On this species the carina is not prominent, and its nodes are nearly 

 obsolete, leaving the margin but slightly wavy. The two upper rows of 

 nodes are present. 



Potamidopsis tricarinata mut. fatua mut. nov. 



1824. Cerithium tricarinatum var. f Deshayes, Desc. des coquilles foss. des 

 environs de Paris, II, p. 326, pi. 51, fig. 9. 



This is an extreme mutation, which is distinguished by the almost 

 entire absence of nodes, in the adult stages, from both the carina and the 

 two spirals above it. The variation is carried so far in this form that it 

 would be described as a distinct species if the nodes had altogether disap- 

 peared. 



Potamidopsis acus Deshayes 



1866. Cerithium acus Deshayes, Desc. des animaux sans vert, decouverts dans 

 le bassin de Paris, III, 199, pi. 75, figs. 19, 20. 



Measubements (Deshayes) : Length, 25 mm. ; greatest diameter, 6 mm. 



Deshayes records the discovery of a single specimen only of this species. 

 According to his figures and description, it has a long, slender spire of 

 twenty-five volutions. The adult is ornamented by two rows of nodes 

 connected by ribs, the lower row being more prominent than the upper. 

 A fine undulating spiral is present just above the suture. 



The aperture is like that of P. tricarinata, except that it does not show 

 gerontic characters in the thickening of the shell and loss of ornamenta- 

 tion seen on many specimens of the former species. 



Deshayes calls attention to the similarity of this shell to the young 

 P. tricarinata, and it might be considered the young of that species were 

 it not that all the specimens of P. tricarinata studied, of 25 mm. in 

 length or of twenty-five volutions, have passed beyond the stage repre- 

 sented by the last whorl of P. acus in the intercalation of a finely nodose 

 spiral between those already existing. If P. acus is actually adult, as it 

 appears to be, it represents a more primitive form than P. tricarinata 

 and may well be the immediate ancestor of that species. This hypothesis 

 would be confirmed if other specimens showing young stages similar to 

 those of the very young P. tricarinata were discovered. 



Horizon and locality : Calcaire grossier. Monchy, Paris Basin. 



