GASTEROPODA, OR UNIVALVES. 21 



ligament long, narrow ; upper margitt angular ; ajiterior side 

 straightish ; posterior side curved ; lower margin incurved ; 

 inside rather pearly ; hinge toothless, strong, with a hollow, 

 triangular plate in both valves under the beaks ; scars faint. 



Inhabits sluggish streams, lakes, and canals, in 

 various parts of this country, as far north as Edin- 

 burgh. It is most abundant in the New River, which 

 supplies a portion of the metropolis with water. 

 Gwyn Jeffreys was informed by Mr. Norman " that 

 he saw immense numbers of the Dreissence, in a living 

 state, lining some of the iron water-pipes which had 

 been taken up in Oxford Street, and that the colouring 

 of the shells was as vivid as if the animal had lived 

 in the light of day." — B.C., vol. i. p. 48. 



CLASS IL— GASTEROPODA,* OR 

 UNIVALVES. 



Body covered in front by a one-lobed mantle; head usually 

 distinct : tentacles four or two ; eyes situated either at the ex- 

 tremity or at the base of the tentacles, or sometimes on separate 

 foot-stalks ; foot a. muscular disc adapted for crawling and, in 

 some instances, for floating ; reproductive system variable ; in 

 some cases each animal is both male and female, but requires 

 fecundation by another individual ; in others the sexes are 

 distinct, each animal being either male or female ; respiratory 

 system consisting either of gills or of organs resembling lungs. 

 Among the aquatic kinds both of these systems occur, but 

 those which are terrestrial are provided with the lung-like 

 organ only. 



Shell usually but not always present, consisting of a single 



Belly-footed. 



