WOOD, PHYLOGENY OF CERTAIN CERITHIIDJE 53 



Eemarks: This species resembles C. cequispnale in the young but 

 diverges from the type of development illustrated by that species in the 

 adult. It may be considered a lateral branch from the main line of evo- 

 lution of Cerithium. 



Cerithium bicarinatum Deshaycs 

 Plate II, fig. 8. 



1824. Cerithium hicarinatum Deshayes, Desc. des coquilles foss. des environs 

 de Paris, p. 356, pi. 53, figs. 14, 15. 



Measurements : Leugtla, 24 mm. ; greatest diameter, 9.5 mm. ; apical angle, 

 30° ; sutural angle, 85.5°. 



The youngest volution preserved on the specimen described is .7 mm. 

 in diameter and is ornamented by two spirals, the lower of which is the 

 stronger. Eibs are absent at this stage. The two succeeding volutions 

 are like the first, except that the upper spiral becomes equal to the lower 

 in strength. On the fourth of the volutions preserved ribs appear, and 

 on the fifth a faint constriction below the suture defines a slight sub- 

 sutural band, which, however, lasts for only two volutions and is not seen 

 on the adolescent and adult stages. The specimen thus far described is 

 a young individual preserving only eight volutions, but the same type of 

 ornamentation persists on full-grown specimens, the adult ornamentation 

 being simply two strong spirals with a long shoulder slope, a concave 

 surface between the two spirals and a short lower slope. The body volu- 

 tion has two spirals below the primary ones. 



The aperture is nearly circular, with a short siphonal canal so widely 

 open that its cross-section does not represent a curve of more than 180°. 

 The callus of the inner lip is narrow and has a slight projection near the 

 posterior end, although a distinct tooth is not formed. The outer lip is 

 broken on all the specimens obtainable, but it was evidently thin and, 

 according to the figure of Deshayes, was not crenulated. 



Horizon and localities: Sables Moyens (Upper Eocenic). Aey-en-Multien 

 and many other localities in the Paris Basin. 

 No. 20144, Columbia University collection. 



Eemarks : The adult of this species corresponds in essential character- 

 istics with the fifth volution of Cerithium lamellosum and with the sec- 

 ond of C. adansoni, and although occurring at a higher horizon than 

 C. lamellosum, it illustrates a more primitive type of development. It 

 represents the persistence of a type of shell which was probably devel- 



