60 ANNALS NEW YORK ACADEMY OF SCIENCES 



with the slight canal of Potamides, and later to the well-developed canal 

 of Cerithium. Hence the ancestor from which both genera are derived 

 must have been more like Potamides than like Cerithium, although the 

 type of Potamides, P. lamarclxi, and other species of the genus occur at 

 higher horizons than many well-developed species of Cerithium. They 

 represent the persistence of a primitive type of structure throughout a 

 long period of geologic time, while Cerithium, though descended from a 

 common ancestor, represents a more rapid evolution of highly specialized 

 forms. 



Potamides lamarcki Brongniart 



1810. Potamides lamarcki Brongniart, Ann. du Mus. Nat. d'hist. naturelle, 



XV, 468, pi. 22, fig. 5. 

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



de Paris. II, 412, pi. 50, figs. 32-34. 

 18(>6. Cerithium lamarcki Deshayes, Desc. des niiimanx sans vert, decouverts 



dans le bassin de Paris. III. 177. pi. 80. figs. 2.5-28. 

 1906. Potamides lamarcki Cossmann, Essais de Paleoconch. Comp.. VII, 103. 



Measurements : Length, 22.8 mm. ; greatest diameter, 6.6 mm. ; apical angle, 

 1!)° ; sutnral angle, 88.5°. 



The protoconch of this species is beautifully preserved on a small indi- 

 vidual of seventeen volutions. The form is similar to that of Cerithium 

 adansoni, and the surface is smooth and shining. It comprises about 

 one and one-half volutions, after which two fine spirals of equal strength 

 appear and remain the only ornamentation of the shell for three volu- 

 tions. On the fifth volution the spirals are crossed by ribs, and for the 

 next four volutioi;s the shell has the characteristic ornamentation of the 

 young Cerithium — that is, an angular outline formed by two equal spirals 

 crossed by ribs. On the ninth volution a third spiral is added just below 

 the suture, and all the spirals form nodes where crossed by the ribs. The 

 median row of nodes lags slightly behind the others, so that the ribs are 

 curved in crossing the volutions. The ornamentation of the adult con- 

 sists of three strong nodose spirals which are rectangular in cross-section 

 and a fine continuous spiral just above the suture. The ribs are faint 

 or obsolete in the adult, and the entire surface of the shell is covered by 

 extremely fine, closely set spirals, which are only visible with a strong 

 lens. Some specimens show gerontic characters in the thickening of the 

 shell and the loss of the nodes on the body volution. The lower slope of 

 this volution bears three spirals in addition to those already mentioned. 



The aperture is nearly circular. The callus of the inner lip is thin 

 and. spread out broadly over the lower surface of the body volution, and 



