I AFFINITIES OF THE LIMNAEIDAE I 9 



remarkable that Svphonaria, which lives at a higher tide level 

 than Gadinia, should retain the gill, while Gadinia has lost it. 



The ultimate affinities of the essentially fresh-water groups, 

 Limnaea, Physa, Chilina, cannot be precisely affirmed. The form 

 of shell in Latia, Gundlachia, and perhaps A'ticylus, may suggest 

 to some a connexion with the Otinidae, and in Chilina, a similar 

 connexion with the Auriculidae. But, in a question of deriva- 

 tion, similarities of shell alone are of little value. It is not a 

 little remarkable, for instance, that we should find a simple patelli- 

 forni shell in genera so completely distinct from one another in all 

 anatomical essentials as Ancylus, Patella, Siphonaria, Propilidium, 

 Hipponyx, Cocculina, and Uiiibrclla. 



Some recent authors, on grounds of general organisation, 

 regard the Limnaeidae and their allies as Opisthobranchs 

 adapted to an aerial life. It is held ^ that the Nudibranchiate 

 (Dpisthobranchs have given birth to the Pulmonata Stylommato- 

 phora or land snails, and the Tectibranchiate Opisthobranchs to 

 the Pulmonata Basommatophora or fresh-water snails. Such a 

 view seems at first sight open to some objection from other views 

 than those which deal simply with anatomy. The Opistho- 

 hranchiata are not, to any marked extent, littoral genera, nor do 

 they specially haunt the mouths of rivers. On the contrary, 

 they inhabit, as a rule, only the very lowest part of the littoral 

 zone, and are seldom found, except where the water is purely 

 salt. In other cases, when the derivation of land or fresh-water 

 genera is fairly well established, intermediate forms persist, 

 which indicate, with more or less clearness, the lines along which 

 modification has proceeded. It has, however, recently been 

 shown that Siphonaria - and Gadinia,^ which have, as has been 

 already mentioned, hitherto been classified as Pulmonata, are in 

 reality modified forms of Opisthobranchiata, which are in process 

 of adaptation to a life partly marine, partly on land. They may 

 therefore be regarded as supplying the link, hitherto missing, 

 between the land Pulmonata and the marine groups from one or 

 other of which the latter must have been derived. The general 

 consensus of recent opinion inclines towards accepting these 

 views, some writers * being content to regard the Pulmonata, as 



1 Kg. Bouvier, Le Natural. 1889, p. 242. 



-' Kohler, Zool. Jahrb. vii. 1893, p. 1 f ; Haller, Arh. Zool. Inst. JVien, x. p. 71. 



3 Plate, SB. kon. Preicss. Ak. Wiss. Bcrl. 1893, p. 959. 



* E.g. Pelseneev, B^lU. S'c. France Behj. xxiv. p. 347 f. 



