52 TESTACEA. 



I am unable to identify this species with the Planorbis rhombem of 

 Turton, p. 108, fig. 90, as he describes the shell as " striolate" with a 

 sharp keel near the base." 



Planorbis caeinatus — Plate XI. Fig. 6. 



Shell lialf iin incli in diameter. Volutes five ; orifice oblique or 

 irregularly elliptical. A sharp keel, commencing at the inner edge of the 

 orifice, runs along the largest volution, near the base. Horn colour, 

 sometimes almost black. Animal a Limax ; body nearly three lines long ; 

 anterior truncate, posterior extremity obtuse. 



One side of the shell is somewhat more concave than the other : the 

 last volute enlarges considerably, so that the orifice expands two lines in 

 its obliquity. The volutions are much striated, perhaps from receiving 

 frequent accessions. The inhabitant feeds on animal and vegetable sub- 

 stances. 



Varieties either want the keel, or it is almost obliterated. 

 The tenant of this shell is apt to assume such a place and position, 

 that, from the disparity of its body compared with the shell, the latter is 

 sometimes applied immoveable, like others, to the side of the glass. If 

 i-etreating so far within the orifice as to be inaccessible by the water, it 

 will revive after a considerable interval of desiccation, on a portion of the 

 volute being removed to admit contact of the element. 



There are certain peculiarities in the structure of this animal which 

 I cannot explain. They do not seem referable to the season of propa- 

 gation. 



This animal feeds on the common pond weed, consuming the jHircu- 

 ilij/iiKi of the leaf. Night is the great season of its activity : when, in 

 a place where the species is abundant, they may be seen so occupied on a 

 single plant. After their depredations, the leaf appears like net-work, 

 and the whole exhibits a fine and pictures(iue object. 



Fig. 6. Planorbis carinatus ; a, IIea<l somewhat enlarged ; 5, Sole somewhat 

 enlarged ; Specimens feeding. 



