84 ANNALS NEW YORK ACADEMY OF SCIENCES 



plicity of its ornamentation and in its retarded ontogeny. It is probably 

 developed from the same bicarinate ancestor from which Cerithium arose, 

 but includes primitive types which persist throughout several periods of 

 geological time. Eocenic species of Potamides form, like Vulgoceri- 

 thium, a compact group the members of which do not diverge strongly, 

 even in the adult stages. 



The genus Tympanotonus is founded upon Murex fuscatus Linne, a 

 species formerly referred to Potamides, and this shell has the typical 

 Potamides young stages, forming the peculiar ornamentation of the adult 

 by developing its median row of nodes into large spines. The genus 

 should be restricted to those forms which are like Potamides in the young 

 and only show divergence in the neanic or adult stages. As thus re- 

 stricted, the genus is a direct descendant from Potamides. 



The genus Potamidopsis,h.a,Ying for its genotype Cerithium tricarinatum 

 Lamarck, is also closely related to Cerithium, as shown by the develop- 

 ment of the genotype. This species forms a bicarinate ornamentation in 

 the same manner as both Cerithium and Potamides (compare plate vii, 

 figs. 7, 8, with plate iii, figs. 9, 10, and plate iv, fig. 9), but it is more 

 accelerated than Potamides. Beyond this stage, Potamidopsis diverges 

 strongly from Cerithium in forming its shoulder angle just above the 

 suture and in having its surface ornamented by rows of nodes. The 

 genus as a whole is less accelerated than Cerithium. 



As a probable ancestor of P. tricarinata we have P. acus, whose adult 

 stages resemble the ninth to the thirteenth volutions of P. tricarinata, 

 and from this species we have developed P. roissyi and P. tuberculosa, 

 whose development is parallel to that of P. tricarinata up to the P. acus 

 stage. After this stage these forms diverge from P. tricarinata by de- 

 veloping the uppermost instead of the lowest row of nodes. 



The ancestor of Cerithium immediately preceding the Jurassic species 

 described probably possessed a bicarinate ornamentation crossed by ribs 

 and a very slightly developed canal. The young stages probably had 

 rounded whorls, with one spiral at first and later two continuous spirals. 

 The collections thus far studied do not furnish sufficient evidence for a 

 statement as to the canalless form from which this primitive ancestor of 

 the genus was derived. 



Cerithium is a genus which shows a strong tendency to vary, as shown 

 by the great diversity of forms present in the Eocenic and still greater 

 variety in recent time; but, notwithstanding the wide differences in 

 adults, relationship may be traced by similarity in the young stages, 

 pointing out the path of evolution which all have traveled. 



