WOOD, PHYLOGEyr OF CERTAIN CERITHIID^ 75 



The aperture is roughly quadrangular. The callus of the inner lip is 

 thick and expanded posteriorly. The siphonal canal is short, twisted 

 and has a strong columellar ridge defining its posterior margin. The 

 outer lip is broken in the specimens studied, but, according to the figures 

 of Deshayes, cited above, it is thin and strongly sinuous. 



Horizon and locality : Oligocenic. Jeures, Paris Basin. 

 No. 20151, Columbia University collection. 



Eemarks : The development of this shell is closely similar to that of 

 P. tricarinata, so far as can be determined from the specimens studied. 

 The adult differs only in the fact that the lowest row of nodes equals the 

 uppermost in prominence, instead of exceeding it. This causes the out- 

 line of the volution to be parallel with the slope of the spire, instead of 

 forming a shoulder. P. conjuncta probably occupies about the same 

 relation to P. tricarinata as does P. tuberculosa, diverging from the tri^ 

 carinata line of evolution at about the same stage in its development, but 

 in a different direction. The adult of P. tricarinata shows greater prom- 

 inence of the lowest row of nodes; that of P. tuberculosa emphasizes the 

 prominence of the sub-sutural row, and in P. conjuncta they are equal. 



Potamidopsis trochleare Lamarck 



1804. Cerithium trocJileare Lamarck, Ann. du Mus. d'hist. naturelle. III, 349. 

 1824. Cerithium trochleare Deshayes, Desc. des coquilles foss. des environs de 



Paris. II. .388, pi. 55. figs. 10, 11. 

 1SG6. Cerithium trochleare Deshayes, Desc. des animaux sans vert, decouverts 



dans le bassin de Paris. Ill, 129, pi. 80, figs. 1-8. 

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



118, pi. 11, fig. 19. 



Measurements : Length, 25 mm. ; greatest diameter, 12 mm. ; apical angle, 

 25.2° ; sutural angle, 85°. 



One of the specimens studied shows the protoconch. This is similar in 

 form to the protoconch of Cerithium adansoni and comprises about one 

 and one-half volutions. The volutions immediately succeeding the proto- 

 conch on this specimen are poorly preserved; but another specimen, 

 whose youngest volution is .7 mm. in diameter, retains the surface feat- 

 ures. This is the third volution, as shown by comparison with a com- 

 plete individual. It is ornamented by two nodose spirals, of which the 

 lower is the more prominent, and a third fine intercalated spiral. This 

 is the ornamentation of the adult P. tricarinata. The median spiral per- 

 sists for two volutions only, after which the shell has two rows of nodes 

 only, with the lower more prominent than the upper. The latter stage 



