58 THE LIFE OF THE MOLLUSCA 



poda and Pelecypoda. The former include some 

 species of the Rhipidoglossate Theodoxis and its relative 

 Septavia; Taenioglossate families such as the Vivip- 

 aridas, Ampullariidae (a completely amphibious clan), 

 Valvatidae, most of the Paludestrinidaa, Melaniidae, 

 Typhobiidae (in Lake Tanganyika), and Pleuroceridae ; 

 Rhachiglossate forms as Canidia, Clea,o.nd Nassodonta, 

 and Pulmonate families as Limnaeidae, Planorbidae, 

 Ancylidae, Physidae, commonly known as Pond 

 Snails (Plate XIIL, Figs. 1-7). The Pelecypoda 

 belong mostly to the Sub-Mytilacea, and include the 

 big family Unionidae, or freshwater Mussels, of which 

 the United States boasts no less than about 530 

 species, or more than half of those known; with 

 their kindred the Mutelidae ; the freshwater Oysters, 

 ./Etheriidae, of the African and American rivers ; the 

 Rangiidae, Cyrenidae, and the interesting Zebra 

 Mussel (Dreissensia, Plate XV., Fig. 9). A fresh- 

 water representative of the Veneracea (Glaucomya), 

 occurs in South-East Africa. 



All these, like their marine confreres, more or less 

 sort themselves according to their surroundings. 



The Limpet-like Ancylus (Plate XIIL, Fig. 1) and 

 Septaria cling to rocks in swift streams. Other 

 Gastropoda hide in the water-weeds, or crawl on 

 the muddy banks. The Bivalves mostly dwell in 

 the mud, but Dreissemia attaches itself 03^ its byssus 

 to rocks, etc., in moving water; and many of the 

 small Cyrenidas (Sphcermm, Plate XVI II., Fig. 16, 



