]4: ANNALS NEW YORK ACADEMY OF SCIENCES 



The aperture is similar to that of P. tuberculosa, but the canal is 

 longer and more strongly reflexed, and the callus of the inner lip is 

 thicker and is not closely applied to the surface of the shell. The outer 

 lip is thickened with crowding of the growth line and loss of ornamenta- 

 tion for some distance back of it. 



HoBizoN AND LOCALiTy : Sables Moyens. Rosoy-en-multiens, Paris Basin. 

 No. 20150, Columbia University collection. 



Kemarks : This species has been regarded as a variety of Potamidopsis 

 tuberculosa, but its much greater size, strongly marked surface features, 

 and longer siphonal canal, as well as its more accelerated development, 

 entitle it to rank as a distinct species. It is evidently derived from the 

 same ancestor as P. tuberculosa, following the same path of evolution 

 but strengthening all the features of the shell. The three species, P. 

 roissyi, P. tuberculosa and P. crassinoda, possess the same type of orna- 

 mentation, and they form a series in which P. roissyi is at one extreme, 

 characterized by small size, thin shell and delicacy of ornament, and 

 P. crassinoda, at the other, characterized by thick shell, strong tubercles 

 and wide apical angle. 



Potamidopsis conjuncta Deshayes 



1824. Cerithium conjunctum Deshayes, Desc. des coquilles fossiles des en- 

 virons de Paris, II, 387, pi. 73, figs. lA. 



1866. CeritMum conjunctum Deshayes, Desc. des animaux sans vert, decou- 

 verts dans le bassin de Paris, III, 123, pi. 80, figs. 9-16. 



1906. Tympanotonus conjunctus Cossmann, Essais de Paleoconch. Comp., VII, 

 120. 



Measurements : Length of specimen from which apex is broken, 29.4 mm. ; 

 greatest diameter, 10.8 mm. ; apical angle, 28.2°, changing to 18.2° on the last 

 three volutions ; sutural angle, 86°. 



The youngest volution available for study is 2 mm. in diameter, and 

 several volutions have been broken away above it. The ornamentation 

 is that of two rows of nodes, one near each suture, the lower row being 

 the more prominent. The upper and lower nodes are connected by ribs. 

 This ornamentation resembles that of P. tricarinata from the ninth to 

 the thirteenth volutions. On the third volution present a fine spiral is 

 intercalated between the two rows of. nodes. This spiral soon becomes 

 finely nodose, and the ornament of the adult is that of three rows of 

 nodes of which the uppermost and lowest are large and of equal size, 

 while the nodes of the median row are very small. The body volution 

 has two strong spirals and several fine ones on its lower slope. 



