70 ANNALS NEW YORK ACADEMY OF SCIENCES 



Potamidopsis mixta Dcshayes 



. Cerithimn mixtum Defrance, Nom. nud. 



1824. Cerithimn mixtum Deshayes, Desc. des coquilles foss. des environs de 



Paris, II, 324, pi. 45, figs. 6-11. 

 18G6. Ceritliium mixtum Deshayes, Desc. des animaux sans vert, decouverts 



dans le bassin de Paris, III, 123. 

 1906. PtycJiopotamides mixtum Cossmann, Essais de Paleoconch. Comp., VII, 



108. 



Measurements : Length of the eleven volutions preserved, 41 mm. ; greatest 

 diameter, 15 mm. ; apical angle, 27.5°, changing to 21.8° on last three volu- 

 tions ; sutural angle, 80°. 



The youngest volution preserved on the shell studied is 3 mm. in diam- 

 eter; hence a considerable portion of the apex has been broken away. 

 The first three volutions are much worn, destroying all the finer surface 

 features, but the fourth has precisely the ornamentation of P. tricarinata 

 mut. hrontes, namely, a strong nodose carina near the base of the volu- 

 tion, a sub-sutural row of rather strong nodes and a median row of 

 nodes much finer than either of the others. This remains the ornamenta- 

 tion of the shell until the third from the last volution, when the sub- 

 sutural and the lowest row of nodes become equal in strength, finer and 

 much crowded. A fourth, finely nodose carina is developed at the base 

 of the volution and is partially concealed by the succeeding whorl. On 

 the body volution the nodes of the upper row are small and in places so 

 close together as to become confiuent. Two nodose carinas and several 

 fine spirals are present on the lower slope of the body volution. 



The aperture is similar to tbat of P. tricarinata, with an expanded 

 callus on the inner lip and a comparatively long, twisted siphonal canal. 

 A slight columellar plication defines the posterior margin of the canal. 

 The outer lip is broken in the specimen at hand, but the figures of 

 Deshayes, cited above, show it to be similar to that of P. tricarinata. 



Horizon and localities : Sables Moyens. Anvers and other localities in the 

 Paris Basin. 



No. 20147, Columbia University collection. 



Remarks : So far as can be determined, the life history of this species 

 is like that of P. tricarinata until the last three volutions, when the orna- 

 mentation changes to that of three rows of crowded nodes, the uppermost 

 being slightly more prominent than the lowest. It is probably a de- 

 scendant of P. tricarinata, showing its divergence from its ancestor on 

 the last three volutions only. 



Tbe columnellar plication which furnishes the reason for placing this 



