6 SPH&RIIDJE. 



4. S. LACUS'TRE,* MULLER. PL. I. 



Body whitish, or of a faint rose-colour ; foot about twice the 

 length of the shell ; mantle greyish at the edges ; siphons long, 

 the respiratory one cylindrical and truncate at its orifice, which 

 is large ; anal siphon slightly conical, orifice small. 



Shell roundish, equilateral, somewhat compressed, very thin, 

 shining, semitransparent, pale greyish horn-colour, occasionally 

 marked with bands of a darker colour, slightly striate concentri- 

 cally ; epidermis thin ; anterior and posterior sides truncate and 

 sloping from the upper side ; front margin sharp and somewhat 

 curved ; beaks central, very prominent, tipped with the nucleus 

 or first growth of the shell ; ligament narrow, faintly visible out- 

 side ; inside bluish-white ; hinge strong, teeth as in S. corneum^ 

 but smaller in proportion ; scars indistinct. 



Inhabits canals, lakes, and ponds in most parts of 

 England, Wales, and Ireland ; in Scotland it has been 

 found near Glasgow. 



Gwyn Jeffreys says he has "found it alive in the 

 hardened mud of a pond which had been drained, and 

 its bed so completely dried up by the sun as scarcely 

 to show the marks of any footsteps on it." B.C., 

 vol. i. p. II. 



This species may be distinguished from the other 

 members of the genus by the rounder form and 

 extreme slenderness of its shell, as well as by the 

 presence of the nucleus on the beaks. 



Var. i. Brochoniana. Shell much larger and flatter ; beaks 

 smaller and less prominent. Clumber Lake, Notts, B.C. Near 

 Hornsey (Rich). 



Var. 2. rotunda. Shell rounder and flatter ; epidermis yel- 

 lowish-green. Singleton, near Swansea, B.C. 



Var. 3. Ryckholtii. Shell small, triangular and globular ; 

 beaks very prominent. Marsh between Exmouth and Budleigh 

 Salterton, B.C. 



* Living in lakes. 



