HELIX. 909 



pressed Helices by their greater thickness in proportion to 

 the size, and by its singularly large spiral umbilicus. 



ALBA. 52. Shell umbilicated on botli sides, with 

 four whirls, and slightly decussated striae ; 

 aperture dilated. 



Helix alba. Gmelin, p. 3625. Montagu Test. p. 459- t. 



25. f. 7. Maton and Racket, in Lin. Trans, viii. p. 192. 



Dorset Cat. p. 53. t. 9- f- 18. 

 Helix, No. 3l6. Schroeter Einl. ii. p. 26?. 

 Planorbis albus. Muller Verm. ii. p. l64. Schroeter Fluss. 



p. 225. t. 5. f. 12. 

 Petiver Gaz. t. 92. f. 7. Martini Berl. Mag. iv. t. 8. f. 23. 

 Inhabits rivers, ditches, and ponds in England. Petiver, 3^c. 



On aquatic plants in Denmark. Muller. 

 Shell about a quarter of an inch in diameter, of a pale horn-co- 

 lour, covered when alive with a brown epidermis ; the apex 

 is depressed into a sub-umbilicus, and the base somewhat 

 convex, and distinctly umbilicated, the whole shell, when 

 slightly magnified, appears to be marked with very fine lon- 

 gitudinal and transverse decussated striae. 



CRYSTALLiNA. 53. Shell with four rounded whirls, 

 rather convex above, and the base largely 

 umbilicated ; aperture nearly orbicular, with 

 a reflected margin. 



Helix crystallina. Muller Verm. ii. p. 23. Gmelin, p. 



S635. 

 Helix paludosa. Da Costa Brit. Conch, p. 59' Montagu 



Test. p. 440. Maton and Racket, in Lin. Trans, viii. 



p. 193. t. 5. f. 5. Dorset Cat. p. 53. 1. 19- f. 25. 

 Variety. With the whirls regularly striated transversely, and 

 the striae membranaceous. 



Helix crenella. Montagu Test. p. 441. t. 13. f. 3. 

 Turbo helicinus. Lightfoot, in Lin. Trans. Ixxvi. t. 3. f. 1 



to 4. 

 Inhabits Denmark among moss. Muller. Boggy places in Eng- 

 land. Da Costa, ^c 

 Shell about one-tenth of an inch in diameter, white, with four 

 well-defined rounded whirls ; Mr. Montagu says that every 

 gradation is to be seen, from the most strongly striated speci- 

 mens of H. crenella, to those of //. crystallina, which are 

 entirely destitute of any such mark. 



