212 PROCEEDINGS OF THE ACADEMY OF [Mar., 



from the associated species by its decidedly broader figure. The 

 diameter of last whorl is contained about 2| times in the estimated 

 total length. The whorls, of which somewhat over 5 are preserved 

 in the type, are shorter and broader than in H. siliceus. Very little 

 of the shell is preserved and the sculpture is unknown. It is proba- 

 bly smooth. The aperture is largely concealed by anotheV shell, 

 the thin, arcuate, outer lip alone remaining visible. 

 Length of broken specimen 17 mm.; diameter 8.5 mm. 



Bythinella antiguensis n. sp. Text fig. 1. 



The shell is oblong, pupiform, smooth; outlines of the spire 

 convex, the apex conspicuously obtuse. Whorls 4, very convex, 

 aperture vertical, shortly ovate, its length contained 2| 

 times in that of the shell- peristome in one plane, thin. 

 Length 1.8, diam. 1.1, length of aperture 0.7 mm. 

 This very minute form is not rare. It has the very 

 obtuse summit and the pupiform shape of the species 

 usually referred to Bythinella, rather than the shape 

 of Paludestrina, if, indeed, the two groups are distinct. 

 Of course, any generic reference of a minute fossil 

 Amnicoloid shell of this sort is purely provisional, unless it is from 

 a region where the recent fauna and its antecedents are well known. 



Planorbis siliceus n. sp. PI. IX, figs, la, 3a, 5a, 6a. 



This is a species of the section Tropicorbis.^ The shell is rather 

 thick, with the periphery rounded, more convex towards the right 

 side. The two sides are about equal in width of the concavity, but 

 that on the right side penetrates more deeply, being vortex shaped. 

 The last whorl is rounded on this side. On the left side the last 

 whorl is subangular and the cavity is less infundibuliform. 



Diameter 3.5, greatest alt. 1.7 mm. 

 5 " " 2.1 " 



This species belongs to a widely spread group of the modern tropical 

 American fauna. 



Paleogeographic Results. 



The species of Hemisifius, as of related genera, are river snails. 

 They do not inhabit intermittent streams, ponds, or lakes, except as 

 the latter may form part of a stream system; and they are equally 

 unknown in brackish water. The occurrence of several well-differen- 

 tiated species therefore implies the presence of rivers or permanent 



5 Tropicorbis n. section, type P. liebmanni Dkr. 



