42 (TiMEHRI. 



posterior end of the broad walking base — the so-called 

 foot of the Moliusca. 



The Melanias, which are widely distributed along the 

 banks of the great rivers, and are especially nume- 

 rous in certain places among the rapids of the 

 Essequibo, are easily distinguished by their elon- 

 gated, and turreted spiral shells, the aperture of which is 

 nearly oval, and pointed above. Often the shells are 

 markedly eroded by the a6lion of the water, and may even 

 become truncated. Melania atra is long, and strongly 

 ribbed ; while M. circumsulcata is shorter and rounded, 

 and not ribbed. 



The Ampullarias are more or less globular, with a 

 small spire, and have the aperture wide and open, cor- 

 responding to the swollen body-whorl. The surface is 

 striated and generally marked with a variable number of 

 bands. They are abundant in weedy ponds and trenches ; 

 burying themselves in the mud during drought, and 

 depositing their egg capsules in little elongated masses 

 upon plants and other obje6ls raised above the water- 

 level. Two species are very widely distributed along 

 the coast, the mere common (Ampullaria ^lauca) 

 having a thick, strong shell, with a wide and open hollow 

 axis to the spire — the axis being thus perforated or 

 umhilicated — and depositing green eggs ; while the less 

 common (A. papyracea ?) has a thin, weak shell, with a 

 very narrow fissured axis — thus being rimate — and 

 depositing pink eggs. The shells of the latter species 

 brought from the shallow ponds of the Rupununi 

 savannah, where the supply of carbonate of lime is 

 evidently very small, are peculiarly thin and brittle. 

 In the well-marked species, A. cornu-arietes, the shell 



