261 



ated in the dead shell." This is caused by the striae 

 being most prominent on the periostraca. 



102. 3. PLANORBIS Icevis. Smooth Coil Shell, (t. 12. 

 f. 148.) Shell rather concave, and whorls equally 

 convex on both sides; brownish horn-colour, 

 semitransparent, smooth or slightly wrinkled by 

 the lines of growth ; with three or four compact 

 and rounded whorls, and a nearly circular aper- 

 ture. 



Planorbis laevis. Alder, Cat. Supp. Trans. Soc. 

 Newcastle. 



Inhab. ponds, north of England, Whitley, North- 

 umberland, and Holy Island. 



This very distinct species approaches nearest to 

 P. albus, but is smaller, has the whorls more rounded 

 and closer set, and is quite destitute of spiral striae. 

 It bears considerable resemblance to Valvata cristata, 

 especially in its young state, and is often covered with 

 a dark incrustation. 



It was discovered by Mr. Alder and the Rev. W. 

 Mark, in Sept. 1832, and first published by Mr. 

 Alder in 1838. It is very like P. spirorbis. 



Mr. Thompson has also found it at Belfast. 



Very like Planorbis parvus of Say, from the United 

 States. 



103. 4. PLANORBIS imbricatus. Nautilus Coil Shell, 

 (t. 8. f. 94.) Shell depressed, thin, pellucid, rather 

 concave above, flattish beneath ; whorls depressed, 

 obtusely keeled, with spinous ridges across the 

 outer whorl ; aperture oval, united all round. 

 Planorbis imbricatus. Mutter, Verm. ii. 165. ; Drap. 

 p. 44. 1. 1. f. 4951. ; Brard, p. 163. t. 6. f. 10, 



