236 LIMN^AD^. 



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 cris- 

 tata, especially in its young state, and is often co- 

 vered 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. spirorhis. 



Mr. Thompson has also found it at Belfast. 



Very like Planorbis parvus of Say, from the 

 United States. 



Mr. Alder described this species as new, because 

 Mr. Jeffreys sent to him a variety of P. alhus as his 

 P. glaber. (See Forbes and Hanley^ Brit. Moll. 151.) 



Mr. Spence Bate describes the teeth as having 

 more strongly serrated margins to their hooks than 

 those of P. albus. 



106. 4. Planorbis nautileus. Nautilus Coil Shell. 

 — Shell depressed, tliin, pellucid, rather con- 

 cave above, flattish beneath ; whorls depressed, 

 obtusely keeled, with spinous ridges across the 

 outer whorl ; aperture oval, united all round, 

 (t. 8. f. 94.) 



Planorbis imbricatus. Midler, Verm. ii. 165.; Drap. p. 44. 

 t. l.f. 49— 51.; Brard, p. 163. t. 6. f. 10, 11.; Jeffreys, 

 Linn. Trans, xvi. 388.; Trirton, Man. ed. 1. p. 11. V. 95.; 

 Gray, Man. 261. t. 8. f. 94.; Leach, Syn. Moll. 115.— 

 Helix nautileus. Linn. S. N. 1241.; Walker, T. M. R. 

 f. 20, 21. ; Mont. p. 464. t. 25. f. 5. — rianurbis nautileus. 

 Flem. J£. Ency. vii. 69. • — Turbo nautileus. Maton and 

 Racket, Linn. IVans. viii. 169. t. 5. f. 4. — Planorbis Nauti- 

 lus. Flem. B. A. 278.; Dnpiii/, Moll. Gers, 51.; Forbes 

 and Hanley, B. M. iv, 152. t. 126. f. 6, 7. 



