CLASS IV 



GASTROPODA 



573 



air. The Pulmonates have, witli few exceptions, no operculiim, and the sliell is often 

 vestigial or absent. 



Next to the Prosobranchs the Pulmonates are the largest group of Gastropods, 

 there being upwards of 6000 living and 700 fossil species known. The most 

 important and highly diversified genera {Helix, Bulimus, Ölausilia) are terrestrial in 

 habit ; certain others {Planorhis, Lymnaea, Physa) are confined to fresh water. The 

 oldest Piilmonates are of rare occurrence in the Devonian and Carboniferous ; they 

 are found sparingly in the Jura and Cretaceous, are of greater abundance in the 

 Tertiary, but do not attain their maximum distribution until the present geological 

 period. 



The Thalassophila and Auriculidae are restricted to marine deposits ; the re- 

 maining Pulmonates are rarely found except in fresh-water strata, and are commonly 

 associated with other organisms that have been swept by rainfall or running water 

 into swamps or estuaries. 



Suborder A. THALASSOPHILA Gray. 



Shell either spiral and operculate, or bowl-shaped to depressed conical, ivithout spire, 

 and somewhat unsymmetrical. Animal usually provided with a single gill in addition 

 to the lung cavity. Tentacles fused with the discoidal head. Eyes sessile. 



The Thalassophila inhabit the littoral zone of the ocean and brackish estuaries. 

 Fossil remains occur from 

 the Devonian onward. 

 Three families are recog- 

 nised — Siphonariidae, Gadi- 

 niidae and Amphibolidae, 

 but these are not readily 

 distinguishable by shell 

 characters alone. 



Siphonaria Blainville 

 (Fig. 1076). Shell usually 

 radially ribbed. Apex 

 directed backwards or to- 

 ward the left side ; inter- 

 nally with two unequal muscular impressions, which are interrupted on the right 

 side in front by a broad groove. Tertiary and Recent. 



Hercynella Kayser (Fig. 1077). Devonian. Anisomyon Meek and Hayden. Jura 

 and Cretaceous. 



Valenciennesia Rousseau. Shell very thin, broadly bowl-shaped, concentrically 

 ribbed. Apex situated near the posterior margin. Right side bearing a broad 

 plication for the respiratory tube. Found in brackish water, Congerian Stage (Pliocene) 

 of Hungary, Roumania and South Russia. 



Williamia Monts. ; Gadinia Gray. Recent and Pliocene. 



Amphihola Schum. Shell spirally globose, thick, rugose and operculate. Recent. 

 This is placed in a separate family, the Amphibolidae. 



Fig. 1077. 



Siphonaria crassicosfata Desh. 

 Eocene ; Anvers, near Paris. 



Herc^jnella bohemica Barr. Devonian 

 (Etage F) ; Lochkow, Bhemia. 



Suborder B. BASOMMATOPHORA A. Schmidt. 



Shell invariably present. Eyes situated at the base of a pair of tentacles. 

 or living in the vicinity of water. 



AquatiCj 



Family 1. Auriculidae Blainville. 



Shell thick, ovate. Spire short, body whorl very large. Inner lip or columella bear- 

 ing plications. Shore forms or inhabitants of sali marshes. Jura to Recent. 



