I AFFINITIES OF THE LIMNAEIDAE 1 9 



remarkable that Siphonaria^ which lives at a higher tide level 

 than G-adinia^ should retain the gill, while G-adinia has lost it. 



The ultimate affinities of the essentially fresh-water gioups, 

 Limnaea^ Physa^ Chilina^ cannot be precisely affirmed. The 

 form of shell in Latia^ Grundlachia^ and perhaps Ancylus^ may 

 suggest to some a connexion with the Otinidae, and in CMlina^ 

 a similar connexion with the Auriculidae. But, in a question of 

 derivation, similarities of shell alone are of little value. It is 

 not a little remarkable, for instance, that we should find a 

 simple patelliform shell in genera so completely distinct from 

 one another in all anatomical essentials as Ancylus^ Patella^ 

 Siphonaria^ Propilidium^ Hipponyx^ Coeculina^ and Umbrella. 



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 Opistho- 

 branchs have given birth to the Pulmonata Stylommatophora 

 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 Opis- 

 thohranchiata 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 SipJionaria'^ 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 accept- 

 ing these views, some writers^ being content to regard the 



1 E.g. Bouvier, Le Natural, 1889, p. 242. 



2 K6hler, Zool. Jahrb. vii. 1893, p. 1 f ; Haller, Arh. Zool Inst. Wien, x. p. 71. 



3 Plate, SB. kon. Preuss. Ak. Wiss. Berl. 1893, p. 959. 



* E.g. Pelseneer, Bull. 8c. France Belg. xxiv. p. 317 f . 



