GASTEROPODA, OR UNIVALVES. 21 



ligament long, narrow ; upper margin angular ; anterior 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 II. 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, 



