LIMN^IDJB. 99 



Yalenciennesia, Rousseau, 1842, 

 Dedicated to the late Professor Yalenciennes of Paris. 

 Distr. — V. annulata, Rousseau. Associated with fresh-water 

 sliells 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. 



Suborder HYGROPHILA. 



Teguments smooth ; living in fresh water and onl}^ 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 Limnsea and Planorbis, composed of thiee pieces 

 corresponding to the three lips of the mouth, and not completely 

 separated. Central and lateral teeth as in Helicidse, 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. Phj'tophagous (Physa is some- 

 times carnivorous). Swimming in a reversed position at the 

 surface of the water. 



Family LIMN^IDJE. 



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 mai"ginals multicuspidate (xiii, 64, 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 i ; 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 Confervse 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 



