CHAP. XXII.] MOLLUSCA. 509 



Family 8.— PYEAMIDELLID^. (10 Genera, 220 Species.) 



DiSTKiBUTiON. — These turreted shells are very widely distri- 

 buted both in temperate and tropical seas; and most of the 

 genera have also a wide range. There are about 400 extinct 

 species, from so far back as the Lower Silurian to the Pliocene 

 formations. 



Family 9.— CEPJTHIAD^. (5 Genera, 190 Species.) 



Distribution. — These are marine, estuary, or fresh-water 

 shells, of an elongated spiral form ; they have a world-wide 

 distribution, but are most abundant in the Tropics. Potamides 

 (41 sp.), is the only fresh-water genus, and is found in the 

 rivers of Africa, India and China, to North Australia and Cali- 

 fornia. Another genus is exclusively fossil, and there are 

 about 800 extinct species, ranging from the Trias to the Eocene 

 and recent formations. 



Family 10.— MELANIAD^. (3 Genera, 410 Species.) 



Distribution. — Fresh-water only : lakes and rivers in warm 

 countries, widely scattered. South Paleearctic and Australian 

 regions, from Spain to New Zealand ; South Africa, West Africa, 

 and Madagascar; United States. There are about 50 fossil 

 species, from the Wealden and Eocene to recent formations. 



Family 11.— TUPtPJTELLTD^.. (5 Genera, 230 Species.) 



Distribution. — Universal. Ccecum is found in north tem- 

 perate seas only. The other genera are mostly tropical, but some 

 species reach Iceland and Greenland. There are near 300 

 species fossil, ranging from the Neocomian to the Pliocene 

 formations. 



