28 
PROCEEDINGS OF THE SCIENTIFIC ASSOCIATION. 
Physa Draparnaud 1801. 
Fluviatile Inoperculata with, smooth thin oval sinisfcral 
shells having a large body-whorl and a small spire. 
Physa rival is Maton and Packett. 
Physa rivalis, Mat. and Rack. Linn. Trans, viii, p. 126, 
pi. 4, f. 2. 
Limnea rivalis, Sowerby. 
Aplexus rivalis, Gray. 
Physa rivalis, Wood, Ind. Test. Bulla 38. 
Physa Sowerbiana, D' Orb. Moll, de Cuba , t. 1, p. 11)0, 
pi. xiii, f. 11, 13. 
A small oval transparent brownish-amber-colored shell 
distinguished from all other Trinidad species by being si- 
nistral. When alive it resembles Succmea approximans (a 
land shell of somewhat similar shape) in being adorned 
with stripes and patches of black which are on the mantle 
of the animal, but are seen through the shell. 
Height 8 mill., diameter 4 mill.* 
Inhabits the Laventille streams with the Plano rb is. 
The eggs of P. rivalis are extremely pellucid and are de- 
posited in jelly-like masses, containing from twenty to forty 
eggs, on objects beneath the surface. This mollusk is very 
fond of gliding along the surface of the water shell down- 
wards, a habit common to most if not all our freshwater 
gasteropoda, not excepting so large a species as Ampullaria 
ejfusa. Physa rivalis also possesses the power of rising sud- 
denly from the bottom to the surface of the water. 
* Since my paper in the “ Annals” was printed I have obtained 
examples of Phjsa rivalis of 12 mill, long and 7 mill, in diameter, 
which bear out the identification with the Cuban shell. 
