166 BRITISH MOLLUSKS. 



range, and is diffused partially and more sparingly up to Scandi- 

 navia. In France, it is partial in its distribution, and becomes 

 scarcer towards the north. Drapamaud made no mention of it in 

 his ' Tableau,' but it was described in the ' Histoire' published after 

 his decease. In Scotland, it is unknown. The latest record of its 

 habitats in England (and local habitats are important in the case of 

 a species like the present) is that given by Mr. Jeffreys : — "North- 

 umberland, Durham, York, Salop, Norfolk, Suffolk, Essex, Oxon, 

 Wilts., Dorset, Cornwall, Guernsey, Jersey, Cork, Belfast," to which 

 I am able to add Surrey and Sussex, on the authority of an able ob- 

 server, Mr. C. H. Gatty, of Felbridge Park, East Grinstead. 



Lymncea glabra, with its slender, cylindrically elongated shell of 

 eight whorls, presents a curious antithesis in form to L. auricularia, 

 with its wide, expandedly inflated shell of scarcely four whorls. 

 The structure of the shells of water snails differs in a most remark- 

 able degree among genera associated in the same locality ; as, for 

 example, of Planorbis, Lymncea, and Ancylus. And wherever the 

 same genera occur in other parts of the globe, they are with little 

 exception typically the same. Descending to species, we have 

 among the Lymncece great contrasts in the plan of convolution, ac- 

 companied by very little difference in details of parts. The growth 

 of L. auricularia is limited to four whorls, and the tubular increase 

 of the shell is rapid ; the mollusk opens out its shell to the utmost 

 degree of expansion, as though striving, so to speak, to make the 

 most of its brief sum of volutions. In L. glabra we have the reverse 

 of these conditions and results. The allotted growth of the species 

 is eight volutions, and the increase is accordingly slow. The shell 

 is drawn out into an elongately subulate spire, not sharply turned 

 as in the first growth of the larger species, which have a largely in- 

 flated whorl to construct on arriving at maturity, but a leisurely 

 constructed equable cylindrical range of whorls, with little or no 

 inflation of the aperture. The parts of the shell in detail — the 

 columella, lip, surface of sculpture, etc. — are the same in both 

 species, and the mollusk is the same dusky slate, black- speckled 

 creature. " L. glabra," says M. Moquin-Tandon, " is a very slug- 

 gish mollusk, carrying its shell a little horizontally, sometimes 

 floating on the surface of the water, sometimes crawling on the 

 sides of a vase out of the water ; at other times it is quite sunk in 

 its shell." 



