PLANORBIS. 89 



thin : periphery slightly keeled towards the lower side, and 

 always angular in consequence of the outward compression 

 of the shell : whorls 6-8, gradually enlarging, the last not 

 exceeding in diameter one-fifth of the whole spire, angular 

 on both sides and sloping gently towards the outer edge : 

 suture well defined, but not deep : mouth obliquely oval, 

 ending above in rather an acute angle, and having the in- 

 side sometimes thickened by a slight rib : outer lip not re- 

 flected : inner lip much spread on the columella, but con- 

 tinuous with the outer lip : umbilicus very large and shallow. 

 L. 0-05. B. 0-3. 



Var. compressa. Shell thinner and much flatter, with the 

 keel more distinct and sharp and placed nearly in the middle 

 of the periphery. P. compressus, Mich. Compl. p. 81, pi. xvi. 

 f. 6-8. 



HABITAT : The same as that of P. spirorbis, and having 

 a similar range at home and abroad as far southward as 

 Algeria. It is, however, not so generally diffused as 

 that species. They are sometimes found together. The 

 present species is also one of our upper tertiary fossils. 

 The variety is not uncommon. I have a distortion from 

 Clumber lake, Notts, in which the spire is displaced on 

 the lower side, and the first whorls are set at an acute 

 angle to those which succeed. 



This mollusk is sluggish, but fond of floating. It lays 

 from 10 to 12 eggs. The epiphragm is thin and mem- 

 branous. 



The shell differs from that of P. spirorbis in being 



-t o 



thinner, flatter, and rather longer, and in having a distinct 

 and prominent keel. This species was first described 

 and figured by Lister. 



8. P. CARINA'TUS*, Muller. 



P. carinatus, Mull. Terra. Hist. pt. ii. p. 175 ; F. & H. iv. p. 153, 

 pi. cxxvii. f. 4, 5. 



BODY deep-reddish brown with a yellowish tint, and of a 

 * Keeled. 



