WOOD, PHYLOGEXY OF CERTAIN CERITHIIDM 63 



EejMarks: Potamides cordieri and its mutation are referred by M. 

 "Cossmann to Ptycliopotamides, a genus which is distinguished from 

 Potamides by the presence of a columellar plication, but such a plication 

 is certainly absent from all of the eleven specimens studied, and the 

 •close similarity in development between this form and P. lamarchi has 

 led to the placing of the species in the genus Potamides. 



Potamides involutum Lamarck 



1804. Cerithiinn involution Lamarck. Ann. du Mus. Nat. d'hist. naturelle de 



Paris. Ill, 348. 

 1824. Cerithiiim involutum Deshates. Desc. des coquilles foss. des environs 



de Paris, II. 328. pi. 41. figs. 10-13. 

 1906. Tytnpanotomus involutum Cossmann, Essais de Paleoconch. Comp., VII, 



120. 



Measueements : Length. 30 mm. ; greatest diameter. 10 mm. ; apical angle, 

 25° ; sutural angle, 87°. 



The youngest volution seen on this species is .8 mm. in diameter. It 

 has at this stage ribs and two equal spirals. The same ornamentation 

 continues for three volutions, and on the fourth a third spiral is intro- 

 duced above the two already existing. This spiral increases gradually 

 in strength until, on the eighth volution, it is as strong as the two below 

 it. At this stage the shoulder has disappeared, and the surface is orna- 

 mented by three equal rows of nodes and a fine continuous spiral just 

 above the suture. On the later whorls, at a stage varying in different in- 

 dividuals, the nodes are lost, leaving the surface marked by continuous 

 spirals only. Still later the two lower spirals also disappear, leaving a 

 single spiral which forms the margin of a strong shoulder angle just 

 below the suture. The extent of this smoothing of the shell varies 

 greatly on different individuals, occupying three or more volutions, or the 

 spirals, and even the nodes, may persist nearly to the end of the body 

 Tolution. Specimens on which the smooth portion comprises several 

 volutions have also a narrower angle of slope for the sides in this portion 

 of the shell, indicating a flattening of the whorls parallel to the axis of 

 coiling. The embracing of the whorls is greater than in the young por- 

 tion of the shell, and on some specimens also a distinct canal is formed 

 at the posterior margin of the aperture by this overlapping of a fold in 

 "the outer lip upon the preceding whorl. 



The aperture is closely similar to that of Potamides lamarcl'i. The 

 siphonal canal is short and widely open, the callus of the inner lip thick, 

 and the margin of the outer lip strongly sinuous. 



Horizon and locality: Calcaire grossier. Cuise-la-raotte, Paris Basin. 

 No. 20155, Columbia University collection. 



