1914.] NATURAL SCIENCES OF PHILADELPHIA. 213 



streams. Melanians are not likely to be distributed by adventitious 

 means from stream to stream, as Physa, Anodonta, and some other 

 fresh-water mollusks are. Their egg capsules are not gelatinous or 

 likely to adhere to the feet of water fowl, but are firmly fixed to 

 stones, shells, or the like. Wherever their distribution has been 

 studied in detail, it has been found remarkably "consistent and 

 explicable by actual stream connections or such as have probably 

 existed within the life of the species or group in question. As the 

 relationships of the Antiguan species are with South America {H. 

 antiguensis) and with Cuba (H. siliceus), it seems probable that (1) 

 at the stage of the Oligocene when this bed was formed, a consider- 

 able land surface existed in the Antiguan area, and (2) that this area 

 was, or had been, connected with the South American main. 



It seems likely that the present Hemisinus species of Cul)a and 

 Jamaica are descendants of the same South American stock. There 

 is in Cuba, however, another totally distinct genus of Melanians, 

 Pachycheilus, represented by P. nigrata (Poey) and P. violaceus 

 Prest. of the recent fauna, which are apparently traceable to a 

 Central American connection. 



The other fresh-water snails of the silex bed are not significant. 

 The Planorbis belongs to a group widely spread in the recent Neotropi- 

 cal fauna and the " Bythinella" is an ambiguous form of unknown 

 relationships.^ 



Explanation of Plate IX. 



Figures 1, 3, 5, 6. — Slabs strewn with Hemisinus antiguensis n. sp. At the posi- 

 tions marked a are seen specimens of Planorbis siliceus n. sp., the type 

 being 6 a. 



Fig. 2. — Hemisinus siliceus n. sp. 



Fig. 4. — Hemisinus latus n. sp. 



* The generic characters of the fresh-water Rissoids are often not expressed 

 in the shell. A fcBsil form of simple structure cannot be located generically with 

 any degree of certainty, unless in a region where the characteristics and affinities 

 of the associated fauna have been thoroughly worked out. 



