1914.] NATURAL SCIENCES OF PHILADELPHIA. , 211 



is but rarely preserved, but in the best examples the peristome seems 

 to be somewhat effuse at the base of the columella. 



Length 16.5, diam. 6 mm., 8 whorls remaining. 

 " 19.5, " 6.4 " 8 



The sculpture of longitudinal ribs with basal spirals is charac- 

 teristic. The same type of sculpture occurs in various South Ameri- 

 can species of Hemisinus. It could readily be matched also in 

 Melania and related forms or in the Pleuroceratidce. 



There seems to be variation in the development of the spirals. 

 Many specimens show weak traces of impressed spirals over the 

 ribs throughout, and this seems to be the normal condition; but in 

 some examples the ribs appear to be smooth except near their lower 

 ends. 



This species, like the associated forms, has the basal sinus or 

 notch obsolete, as in part of the recent species. 

 Hemisinus siliceus n. sp. PI. IX, fig. 2. 



The shell is Melaniiform, regularly tapering, the diameter of last 

 whorl contained about 2^ times in the total length. The whorls 

 are convex, and apparently without any sculpture except growth- 

 lines. The last whorl has fine, reversed sigmoid growth striae, which 

 retract somewhat below the suture, then advance, as in H. cuhaniana. 

 In the type specimen a former peristome, indicating a period of 

 growth arrest, appears as a sigmoid varix on the last whorl. This 

 indicates a more strongly sigmoid outer lip than in the recent Antil- 

 lean species. 



Length 26 mm., about 6 whorls remaining, the summit lost; diam. 

 10.8 mm. 



No entirely perfect aperture was found on the slabs collected, 

 but so far as we can judge, it seems to be much like that of Hemisinus 

 cubanianus (Orb.). It is not unlikely that H. siliceus is ancestral, 

 or at least a collateral species not far removed from the ancestral 

 stock of the smooth Antillean species of Hemisinus. 



There is, of course, a possibility that this Antigua species belongs 

 to the genus Pachycheilus, which is represented in the recent fauna 

 of Cuba by P. conicus (Orb.) and P. violaceus Prest.; but the 

 straighter columella does not, in our opinion, favor this view. 



The type has lost the shell from the spire by conversion into flint, 

 but the surface has been preserved in perfection on the last whorl. 



Hemisinus latus n. sp. PI. IX. fig. 4. 



This form is represented by somewhat numerous internal casts, 

 of which the largest has been selected for illustration. It differs 



