36 THE MOLLUSC A 



northern and southern parts of the same ocean or in several 

 different oceans. Thus Limopsis aurita, Semele profundorum, 

 Verticordia deshayesiana, Area pteroessa are found in the Atlantic 

 and Pacific ; Hyalopeden pudicum and Siknia sarsii are common to 

 the Atlantic and Indian oceans. Characteristic abyssal genera are 

 Leptochiton, Scissurella, Margarita, Cydostrema, Setia, Leda, Bathyarca, 

 Limopsis, Hyalopecten, Dacrydium, Callocardia, and Septibranchs in 

 general. 



C. The Pelagic Fauna. a. The superficial forms are very 

 widely distributed, but may nevertheless be described as belonging 

 to polar and tropical provinces. The North and South Polar fauna 

 are different : to the former belong Clione limacina, Limacina helicina ; 

 to the latter Spongiobranchaea australis, Limacina antarctica. /3. The 

 zonary or deep forms are probably more universally distributed, 

 but are still imperfectly known ; they include the luminous 

 Cephalopoda. 



II. Land and Freshwater Molluscs. It is only a small number 

 of groups that have quitted the sea to assume a freshwater or 

 terrestrial existence, and among these no Amphineura nor Scapho- 

 poda nor Cephalopoda are found. Among the Lamellibranchs only 

 a few families are found in fresh water, viz. Cyrenidae, Dreissensiidae, 

 Unionidae, Aetheriidae (none of them primitive in organisation), and 

 a few isolated types. Among the Gastropods we find very few 

 Rhipidoglossa, Neritina, Hydrocaena, Titiscania ; chiefly Taenioglossa, 

 e.g.ihe Valvatidae,Paludinidae,Ampidlariidae, Hydrobiidae, Melaniidae ; 

 some isolated types of Rachiglossa, and practically no Opistho- 

 branchs. As for the terrestrial Mollusca, there are only a few 

 families of streptoneurous Gastropods (Helicinidae, Cyclophoridae, 

 etc.), and the whole order of Pulmonata. Of the last-named, one 

 sub-order, the Basommatophora, has returned to an aquatic life, 

 chiefly in fresh water, but retains for the most part a pulmonary 

 respiration. 



It is mostly in warm regions, and particularly in those in 

 which the sea is inclined to be brackish, that marine forms have 

 penetrated into fresh waters. Certain inland seas also have 

 become separated from the ocean, and have preserved a fauna 

 which is partly of marine origin. Lakes Baikal and Tanganyika 

 appear to belong to this category. 



The tropical regions in general are characterised by terrestrial' 

 forms, such as Vaginula, Helicina, Ampullaria, etc. Chilina and 

 Bulimus belong to the Neotropical region. Clausilia is not found 

 in North America, and, generally speaking, the Pulmonates with 

 folded branchiae are absent from the New World. The Ethiopian 

 province is the home of Achatina ; the Australo-Zelandic of Janella, 

 Rhytida, Vanganella, Latia ; the Oriental region of Cyclophorus and 

 the Rathouisiidae. The distribution of terrestrial and fluviatile 



