WOOD, PHYLOGENY OF CERTAIN CERITHIIDAJ] 61 



the outer lip is thin and strongly sinuous. The anterior canal is very 

 short, shallow and has a reflexed margin. 



Horizon and locality : Oligocenic. Aurillac, Paris Basin. 

 No. 20153, Columbia University collection. 



Eemarks : This species follows the same path of evolution as Ceri- 

 thium adansoni for the first nine volutions, but is more retarded in the 

 development of ribs than that species. After the ninth volution the 

 shoulder is lost, and the species diverges from the Cerithium line of evo- 

 lution. The adult has flattened volutions and an ornamentation of rows 

 of nodes instead of the angular outline of the whorls and several orders 

 of spirals characteristic of Cerithium. 



As compared with Eocenic species of Cerithium — C. aquispirale, for 

 example — Potamides lamarcTci is more primitive in structure, having a 

 less well-developed canal and simpler type of ornamentation, and it rep- 

 resents, as noted in connection with the genus, the persistence of a primi- 

 tive form while more rapid evolution was taking place in related groups. 



Potamides cordieri mut. typum n. mut. 



1824. Cerithium cordieri var.' a Deshayes, Desc. des coqullles foss. des en- 

 virons de Paris, II, 33S, pi. 52, fig. 8. 



Measurements : Length. 29.8 mm. ; greatest diameter, 11.5 mm. ; apical angle, 

 25° ; sutural angle, 84.5°. 



The youngest volution present is .6 mm. in diameter and is ornamented 

 by ribs and two equal spirals. The same ornamentation continues for 

 four volutions, and on the fifth a nodose spiral appears just below the 

 suture. This spiral increases in strength and becomes more distinctly 

 nodose until, on the eleventh of the volutions preserved, it is as strong 

 as the two nodose spirals below it. At this stage the general outline of 

 the volution is straight and parallel with the slope of the spire, and its 

 surface is ornamented by three equal rows of nodes without ribs. The 

 same ornamentation is continued until the fourteenth volution, when the 

 loose coiling reveals a continuous spiral just above the suture. On the 

 fifteenth volution present a fine, slightly nodose spiral appears between 

 the two upper rows of nodes. The same kind of ornamentation contin- 

 ues throughout the remaining whorls. The body volution has three rows 

 of strong nodes — one fine intercalated row and two strong continuous 

 spirals below the ornamentation of nodes. On the later part of this volu- 

 tion the growth lines are crowded, the shell somewhat thickened and the 

 nodes indistinct or obsolete. 



