WOOD, PHYLOGEXY OF CERTAIN CER[THIID/E 67 



appear on the adult individual until the sixteenth volution, after vrhich 

 it becomes stronger and soon breaks up into a row of fine nodes placed 

 half way between the t^'o rows already formed. The ribs become discon- 

 tinuous, and the last seven whorls are ornamented by three rows of nodes, 

 of which the lowest forms a projecting shoulder angle just above the 

 suture, and the median row is very slightly finer than the upper one. On 

 the later part of the body whorl all the nodes become indistinct or obso- 

 lete, where the crowding of the growth lines and thickening of the shell 

 indicate old-age conditions. 



The aperture is about equal in length and width and somewhat angular 

 in outline. The callus of the inner lip is very thick and broad, its pos- 

 terior part often spreading out for a considerable distance over the pos- 

 terior part of the body whorl. There is no trace of posterior tooth, as in 

 Ceritliium sens. str. The outer lip is often greatly thickened, strongly 

 flaring at the margin, and it overlaps more or less upon the preceding 

 whorl. The thickening of the shell, the loss of ornamentation and the 

 encroaching of the later part of the body whorl upon the preceding whorl 

 are all old-age features which indicate that the species is approaching its 

 extinction. 



Horizon and localities: Calcaire grossier, Sable Moyens. Grignon and 

 many other localities in the Paris Basin. 

 No. 20122, Columbia University collection. 



Eemarks : The young stages of this species furnish a clue to its devel- 

 opment from a form resembling Cerithium hicarinatum, since its devel- 

 opment for the first four volutions is closely parallel to the development 

 of that species, and the fourth volution (counting the protoconch as one) 

 is the counterpart of the adult C. hicarinatum. The species could not, 

 however, have been developed from C. licarinatum itself, since it occurs 

 at an earlier horizon, but was probably developed from the pre-Jurassic 

 ancestor of C. hicarinatum mentioned in connection with that species — 

 the same ancestor which probably gave rise along a different path of 

 evolution to C. corallense and its descendants, C. cequispirale, C. tubero- 

 sum, etcetera. 



Potamidopsis tricarinata has developed many mutations by the accentu- 

 ation or suppression of one or another of its surface features. Of these 

 mutations, Lamarck has described one and given it the designation "P," 

 and Deshayes has indicated five others by letters from '%'' to "f," the 

 letter "a" being used for the type species. The mutations occur at the 

 same horizon and localities as the type, and, as might be expected, transi- 

 tional forms exist between all of them. 



