CHAP. XXII.] MOLLUSCA. £35 



Family 9.— TRTDACNID^. (I Genus, 8 Species.) 



Distribution. — The Tridacnidte, or Clam-shells, are of very 

 large size, and are confined to the Tropical regions of the Indian 

 and Pacific Oceans. A few species have been found fossil in the 

 Miocene formation. 



Family 10.— CARDIAD^. (1 Genus, 200 Species.) 



Distribution. — The Cardiadse, or Cockles, are of world-wide 

 distribution. Another genus is fossil, and nearly 400 fossil 

 species are known, ranging back to the Upper Silurian forma- 

 tion. 



Family 11.— LUCINID^. (8 Genera, 178 Species.) 



Distribution.— The Lucinidae inhabit the Tropical and Tem- 

 perate seas of all parts of the world ; but the genus Corhis is con- 

 fined to the Indian and Pacific Oceans, Montacuta and Lcpton, 

 to the Atlantic. There are nearly 500 extinct species, ranging 

 from the Tertiary back to the Silurian formation. 



Family 12.— CYCLADIDzE. (3 Genera, 176 Species.) 



Distribution.— The Cycladidie are small fresh- or brackish- 

 water shells found all over the globe. The genus Cijdas is most 

 abundant in the North Temperate zone, while Cijrena inhabits 

 the warmer shores of the Atlantic and Pacific, but is absent from 

 the West Coast of America. There are about 150 species fossil, 

 rangino; back from the Pliocene to the Wealden formations. 



Family l;3.-CYPPJNID/E. (10 Genera, 176 Species). 



Distribution. — Universal. Cijimna and Astarte are Arctic 

 and North Temperate ; Cardita is Tropical and South Temperate. 

 There are several extinct genera and about 1,000 species found 

 in all formations as far back as the Lower Silurian. 



