492 



MOLLUSCA 



SUB-KINGDOM VI 



Order 2. PULMONATA. Cuvier. Air-breathing Snails. 1 



Euthyneura in which the gill cavity is transformed into a lung for breathing free air. 

 Mainly terrestrial or fresh-water forms. 



A few Pulmonates have reverted to exclusively aquatic habits, and have the lung 

 filled with water ; and in a few, secondary gills are developed in the cavity. These, 

 however, are rare exceptions. The great majority of forms breathe air by means of a 

 network of blood-vessels spread upon the inner surface of the lung. The ordinary 

 aquatic forms come to the surface of the water at intervals to renew their supply of 

 air. They have, with few exceptions, no operculum, and the shell is often vestigeal 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, Clausilia} are terrestrial in 

 habit ; certain others (PlanorMs, Limnaea, Physa) are confined to fresh water. The 

 oldest Pulmonates 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 ; remaining 

 Pulmonates are rarely found outside of fresh-water strata, and are commonly associated 

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

 into swamps or estuaries. 



Sub-Order A. THALASSOPHILA. Gray. 



Shell either spiral and operculate, or bowl-shaped to depressed conical, without 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 recognised Siphonari- 



idae, Gadiniidae, and Am- 

 phibolidae ; but these are 

 not readily distinguishable 

 by shell characters alone. 



Siphonaria, Blainville 

 (Fig. 1027). Shell usually 

 radially ribbed. Apex 

 directed backwards or to- 

 ward the left side; internally 

 with two unequal muscular 

 impressions, which are in- 

 De- terrupted on the right side 

 in front by a broad groove. 

 Tertiary and Recent. 

 Anisomyon, Meek and Hayden. Jura 



FIG. 1027. 



Siphonaria crassicostata, Desh. 

 Eocene ; Anvers, near Paris. 



FIG. 1028. 



Hercfinclln bohemica, Barr. 

 vonian (Etage F) ; Lochkow, 

 Bohemia. 



Devonian. 



Hercynella, Kayser (Fig. 1028). 

 and Cretaceous. 



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

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



1 Sandberger, F., Land- itnd Siisswasser-Conchylien der Vorvvelt. 1870-75. 

 ]\'/>ite, C. A., Review of American non-marine Mollusca. 

 Tryon, G. W. and Pilsbry, H. A., Manual of Conchology, Pulmonata. 



