36 THE MOLLOUSCA 
northern and southern parts of the same ocean or in several 
different oceans. Thus Limopsis aurita, Semele profundorum, 
Verticordia deshayesiana, Arca pteroessa are found in the Atlantic 
and Pacific; Hyalopecten pudicum and Silenia sarsii are common to 
the Atlantic and Indian oceans. Characteristic abyssal genera are 
Leptochiton, Scissurella, Margarita, Cyclostrema, Setia, Leda, Bathyarca, 
Limopsis, Hyalapecten, 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 Spongiobranchaca australis, Limacina antarcticu. B. The 
zonary or deep forms are probably more universally distributed, 
but are still imperfectly known; they include the luminous 
Cephalopoda. 
Il. 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, Dreissensiidue, 
Unionidae, Actheriidae (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.the Valvatidae, Paludinidae, Ampullarudae, Hydrobudae, Melanidae ; 
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, ete. 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 
