58 THE LIFE OF THE MOLLUSCA 
poda and Pelecypoda. The former include some 
species of the Rhipidoglossate Theodoxis and its relative 
Septaria; Tzenioglossate families such as the Vivip- 
aride, Ampullariide (a completely amphibious clan), 
Valvatide, most of the Paludestrinidz, Melaniide, 
Typhobiide (in Lake Tanganyika), and Pleuroceride ; 
Rhachiglossate forms as Canidia, Clea,and Nassodonta, 
and Pulmonate families as Limnzida, Planorbide, 
Ancylide, Physide, commonly known as Pond 
Snails (Plate XIII., Figs. 1-7). The Pelecypoda 
belong mostly to the Sub-Mytilacea, and include the 
big family Unionide, 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 Mutelide; the freshwater Oysters, 
Etheriidz, of the African and American rivers; the 
Rangiide, Cyrenide, 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 confréres, more or less 
sort themselves according to their surroundings. 
The Limpet-like Ancylus (Plate XIIL., Fig. 1) and 
Septavria 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 Dreissensia attaches itself by its byssus 
to rocks, etc., in moving water; and many of the 
small Cyrenide (Spherium, Plate XVIII., Fig. 16, 
