ANCYLUS. 69 



number, have long recurved hooks, and are at first 

 simple, but becoming ultimately narrowed and 

 minutely toothed; there are 120 transverse rows." 

 In A. laciistris "the central part of each row is much 

 arched, and composed of a central tooth with twelve 

 similar laterals on each side, next to which is a tooth 

 of a different form, and lastly, six more on each side, 

 which latter are in a slight curve ; the number of 

 transverse rows is seventy-five ; the total number of 

 teeth is 2925." 



A. Body sinistral ; shell dextral. 

 I. Ancylus fluvia'tilis,* Muller. Pl. v. 



Body more or less transparent, slaty-grey, finely spotted with 

 black ; tentacles greyish-white, rather transparent, diverging, 

 broad at their base, gradually narrowing to a bluntish tip ; eyes 

 apparent, but small, round, black ; foot yellowish-brown, much 

 expanded at the sides, rounded in front, convex and obtuse 

 behind. 



Shell hood-shaped, moderately thin, of a dullish aspect, pale 

 horn-colour with a yellowish or greenish tinge, with rather 

 strong, regular striae, which radiate from the apex towards the 

 margin of the aperture, and more finely striate in the line of 

 growth ; epidermis thin ; spire recurved or bent downwards 

 towards the posterior margin, apex rather blunt and usually 

 more or less twisted to the right \ mouth oval, margin mem- 

 branous, scarcely reflected. 



Inhabits rivers, lakes, and ponds throughout the 

 British Isles. This little mollusc usually attaches 

 itself, after the manner of the marine Limpets, to 

 stones, rocks, wooden piles, or other submerged 



* Living in rivers. 



