172 BRITISH MOLLUSKS. 



arising out of differences of latitude and the corresponding residts 

 of the law of correlation of growth. G-erstfeldt describes a Siberian 

 species, A. Sibiricus, which is very like A. fluviatilis, but it is of a 

 remarkably conoid form. 



2. Ancylus lacustris. Lake Ancylus. 



Shell ; oblong, rather depressed, compressed at the sides ; very 



thin, transparent, smooth, with the vertex a 



little posterior to central, hooked obliquely <2^ 



to the left. 

 Patella lacustris, Linnaeus (1758), Syst. Nat. 10th edit. 



p. 783. 

 Ancylus lacustris, M tiller (1774), Verm. Hist, part ii. 



p. 199. 

 Patella oblonga, Lightfoot (1786), Phil. Trans. Roy. 



Soc. vol. lxxvii. p. 168. pi. iii. 

 Acroloxus lacustris, Beck (1838), Ind. Moll. p. 124. 

 Velletia lacustris, Gray (1840), Turt. Man. p. 50. f. 126. 

 Ancylus Moquinianus, Bourguignat (1853), Journ. Conch, p. 197. pi. vi. f. 9. 

 Ancylus (Velletia) lacustris, Moquin-Tandon (1855), Hist. Moll. vol. ii. p. 



488. pi. xxxvi. f. 50 to 55. 

 Hob. Throughout Europe. (Adhering to steins and leaves of plants in 



ponds, lakes and canals.) 



A. lacustris is not only distinguished from A. fluviatilis, in 

 having the principal vital organs on the right side of the animal, 

 and the vertex of the shell turned to the left, but in being of a 

 slighter flatter growth, devoid of radiating strise, with the vertex 

 not far posterior to the centre. There is also a difference in the 

 habit of the two species. A. fluviatilis mostly adheres to stones 

 in gently running brooks ; A. lacustris mostly adheres to the 

 stems and leaves of plants in still waters, in ponds, canals, lakes. 

 It is not without reason, therefore, that the genus Ancylus has been 

 subdivided into two. 



As in the case of Ancylastrum so in Velletia, there is to my 

 mind only one European species, and they have much the same 

 geographical distribution; Ancylastrum is, however, the scarcer of 

 the two. The animal is of rather a lighter colour, and of a more 

 livid hue. 



