THE NAUTILUS. 63 



P. glaphyrus var. between polygonatus and immanis. (PI. Ill, figs. 5, 6.) 



The two specimens figured are of the same size but differ in sculpt- 

 ure. One (fig. 6) is smooth above and below, having a strong sub- 

 spinous keel at the periphery, and a smooth, acute keel below it. 

 Upon the earlier whorls of the spire there are longitudinal waves, 

 and two spiral cords above the peripheral keel, Avhich diminishes 

 in size. The base has no spirals. The other specimen has the 

 entire body-whorl spirally lirate (lirre on body-whorl 9, on penulti- 

 mate whorl 3) and strongly plicate. 



P. glaphyrus potamarchus. (PI. Ill, tig. 7.) 



This is one of the largest forms of Pachychilus known, and it is 

 the most aberrant of the glaphyrus stock. The shell is rather 

 slender and acutely conical, the outline of the spire beiug straight. 

 The aperture is ovate, narrowed above, and one-third the length of 

 the shell. Whorls 10-11 remaining, several of the earlier being 

 lost by erosion. The microscopic sculpture is the same as in var. 

 Rovirosai. There are no traces whatever of the waves or folds so 

 prominently shown by the other varieties of glaphyrus, and the 

 spiral cords are also completely obsolete, or indicated by the faintest 

 traces on the base. The color is olive-green or olive-brown. 



Alt. 99, diam. 33 mill. 



Alt. 87, diam. 29 mill. 



Tabasco, Mexico. 



This variety differs from the pyrainidalis of Morelet in being 

 larger and smoother, lacking altogether the chestnut colored spirals 

 of that form. 



Potamanax subgen. nov. 



Shell solid, oval with short conic sjiire, spirally sculptured or 

 banded. Aperture ovate, acute above, broadly rounded below ; 

 outer lip not sinuous; inner lip more or less heavily calloused, not 

 notched at the base. Operculum few-whorled, with basal nucleus. 

 Type P. Rovirosai Pils. 



This group has the sculpture of Hemisinus but differs from that 

 genus in the entire, un-notched basal lip. The columella callus is 

 much like some species of Pachychilus but the operculum is very 

 different from that genus. From both of these groups it differs in 

 the short, ovate contour of the shell. The description of the oper- 

 culum is taken from Melania brevis d'Orbigny of Cuba, which I 

 consider congeneric. 



The relationship of Potamanax to Hemisinus in sculpture and 

 operculum is obvious, and has caused ifle to regard it as an subgenus 



