LIMN IDA. 99 
V ALENCIENNESIA, Rousseau, 1842. 
Dedicated to the late Professor Valenciennes of Paris. 
Distr.— V. annulata, Rousseau. Associated with fresh-water 
shells in a tertiary deposit near Kertch, Crimea. 
Shell resembles a gigantic Ancylus; apex much incurved ; 
surface concentrically marked. A longitudinal plication extends 
from the apex to the right side of the posterior border, and 
corresponds with an internal channel; there is a second but less 
distinct plication on the left side. 
SuporpER HYGROPHILA. 
Teguments smooth; living in fresh water and only coming to 
the surface occasionally to renew their supply of air. ‘Tentacles 
contractile, with eyes at their base. Jaw simple in Physa. and 
compound in Limnzea and Planorbis, composed of three pieces 
corresponding to the three lips of the mouth, and not completely 
separated. Central and lateral teeth as in Helicidz, marginals 
pectinate or serriform. 
Male orifice near the tentacle, female at the base of the neck, 
near the respiratory opening. Eggs contained together in a 
gelatinous, transparent capsule. Embryos without velum, and 
undergoing but slight changes. Phytophagous (Physa is some- 
times carnivorous). Swimming in a reversed position at the 
surface of the water. 
Famity LIMN “ID 4. 
Shell thin, horn-colored, mostly spiral, sometimes patelliform, 
capable of containing the entire animal contracted; aperture 
simple, rounded ; lip sharp. 
Lingual membrane armed with numerous quadrate teeth, 
arranged in transverse rows, the central minute, the laterals 
uncinated, the marginals multicuspidate (xiil, re 65). Head 
with a broad, short muzzle dilated at the end; mouth with a 
horny upper jaw, composed of three pieces, the central much 
the largest (xiii, 66.; tentacles flattened or filiform, with the 
eyes sessile at their inner bases. Mantle-margin variously mod- 
ified, respiratory orifice at the right side. Foot flattened, lanceo- 
late or ovate. 
The fresh-water, air-breathing mollusks of which this family 
is composed inhabit the rivers, ponds, and running streams 
in all parts of. the globe, being, however, most numerously 
represented in temperate regions. They feed on Conferve and 
other aquatic plants. Although usually to be seen crawling on 
the muddy bottoms and on the stems and foliage of submerged 
vegetation, they come to the surface to respire ‘the free air, and 
sometimes may be observed gliding, shell downwards, on the 
