TERTIARY MOLLUSCA. 119 



The following is the original description of this species: 



"The shell is slender, diameter contained nearly three times in the length; 

 whorls rather numerous, probably at least fifteen in a perfect shell, as a young 

 one 12.5 mm. long has twelve whorls, the upper part of the spire being very- 

 slender. Whorls convex, sculptured with many rounded ribs, as wide as their 

 intervals, somewhat curved, the concavity forward and somewhat protractive. 

 There are about 25 ribs on a whorl. Above the lower suture of each whorl 

 there are two or three spiral cords, the lowest one strongest. On the last 

 whorl the ribs extend to the periphery, where they disappear, the peripheral 

 region and the base having numerous spiral cords. The aperture 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 " " 



" The sculpture of longitudinal ribs with basal spirals is characteristic. The 

 same type of sculpture occurs in various South American species of Hemisinus. 

 It could readily be matched also in Melania and related forms or in the 

 Pleuroceratidae. 



"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 obso- 

 lete, as in part of the recent species." 



The revolving threads on the posterior part of the whorl are coarser 

 and more persistently present than one would infer from the description. 

 Locality. Dry Hill Point, Antigua (station 6867), Vaughan. 

 Geologic horizon. Oligocene. 

 Type. Philadelphia Academy. 

 Figured specimen. U. S. Nat. Mus. No. 166995. 



Hemisinus siliceus Brown and Pilsbry. 



(Plate 3, Figure 3.) 

 Hemisinus siliceus Brown and Pilsbry, Proc. Acad. Nat. Sci. Phila., p. 211, plate 9, fig. 2, 1914. 



The following is the original description of this species : 



"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. cubiana. 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 

 Antillean species. 



"Length 26 mm., about six 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 



