236 
LIMN^AD^. 
P. alhusj but is smaller, has the whorls more rounded 
and closer set, and is quite destitute of spiral strise. 
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. spirorlis. 
Mr. Thompson has also found it at Belfast. 
Very like Planorhis parvus of Say, from the 
United States. 
Mr. Alder described this species as new, because 
Mr. J effreys sent to him a variety of P. albus 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. Planokbis nautileus. Nautilus Coil Shell. 
— Shell depressed, thin, 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. Muller^ Verm. ii. 165. ; Drap. p. 44. 
t. 1. f. 49 — 51.; Brordy p. 163. t. 6. f. 10, 11.; Jeffreys^ 
Linn. Trans, xvi, 388.; Turton^ Man. ed. 1. p. 11. f. 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. — Planorbis nautileus. 
Flem. B. Ency. vii. 69. *— Turbo nautileus. Maton and 
Rackety Linn. Imrans, viii. 169. t. 5. f. 4. — Planorbis Nauti- 
lus. Flem. B. A. 278.; Dupnpy Moll. Gers.^ 51.; Forbes 
and Hanley^ B. M. iv. 152. t. 126. f. 6, 7. 
