FAMILY LYMN.EACEA. 155 



difference in the structure and composition of the shell. It is ex- 

 tremely variable in form, sometimes sharply fusiform, sometimes 

 rotundately inflated, and it is of a dull horny striated or malleated 

 texture, never vitrified to anything approaching a porcelain gloss. 

 This remark applies, however, only to the Lymncea proper, of 

 which we have six species in Britain. The two remaining species, 

 L. glutinosa and involute/,, have an extremely thin bright trans- 

 parent shell, covered almost entirely by a reflected extension of 

 the mantle ; and many authors consider them entitled to rank as 

 a separate genus, Amphi/p&plea. 



Lymncea is represented in its distribution over the globe in three 

 very distinct forms. In India, neighbourhood of Calcutta, the shell 

 is of a characteristic cylindrically oblong form. In the Malayan 

 islands and Punjaub districts of India, it is of a peculiar silvery- 

 horny substance marked with opake-white linear streaks. In 

 Western Asia north of the Himalayas, over the whole of Europe 

 extending to Greenland, and over all the United States, the Lymncece, 

 excepting L. glabra and the section Amphipeplea, produce a dull 

 horny malleated shell, such as we have already described. The 

 inland waters of Central America and Australia have few Lymncece ; 

 they are chiefly inhabited by Physce. In the Patagonian and Chi- 

 lian waters of South America the two genera are mainly dispensed 

 with in favour of another type peculiar to that locality, CJiilina. 



The Lymncece of Western Asia and Europe are plentiful in indi- 

 viduals, but comparatively lim ited in species ; and, unlike other 

 Caucasian genera, which diminish in species as they are further re- 

 moved from the central area of the general molluscan fauna, they 

 all pass into Britain. Other species have been described by Conti- 

 nental authors, but they do not appear to me to be tenable. 

 Another peculiarity in the distribution of the Caucasian Lymncece is, 

 that nearly all the species appear to pass in a modified form into 

 the United States. The species of the two countries, if not identical, 

 present a marked degree of parallelism, — L. limosa with L. cata- 

 scopium, L. auricidaria with L. macrostoma, L. stagnalis with L. 

 jugularis, L. palustris with L. elodes, and L. truncatula with L. 

 desidiosa. Our L. glabra and the two Amphipeplea, L. glutinosa 

 and involuia, have no representative in America ; and, conco- 

 mitant with this, there is a pecuharity in their geographical dis- 

 tribution in Europe. Like Physa hypnorum, they are not found 

 south of the Pyrenees. L. involuta, it should be mentioned, is a 

 doubtful species, perhaps a variety of L. glutinosa in which the 



