46 BIVALVES. — MA.CTRA. 



and have a semipellucid appearance. They are, for the 

 most part, thin, brittle, and remarkably light. 



The most prevailing colour is blueish or yellowish-white, 

 but some have, upon a brown ground, delicate rays of pur- 

 ple, heightened with rich tints of the same colour : others, 

 again, are of a brilliant lilac, passing into a delicate blue. 



The hinge of the Mactra is its best distinction from all 

 the other genera of Bivalves, for the middle tooth is almost 

 invariably complicated, and of a triangular form, hav- 

 ing a small hollow on each side ; the lateral teeth are re- 

 mote from the beaks and inserted into each other. The 

 hinge, though very articulate, is remarkably thin and deli- 

 cate; in some cases, the teeth which compose it are much 

 thinner than paper. 



The Mactrse are mostly equivalves ; in a few specimens 

 the valves gape at both ends, and in others at the anterior 

 only; the interior margin is rarely crenated or toothed. 



The Northern and European seas supply many of the 

 species of Mactra?. The Indian and American oceans, the 

 Mediterranean, the shores of Africa and the Cape of Good 

 Hope also produce them. They are also frequently found 

 at the mouths of rivers. 



MACTRA— Kiieading-trongh. 



Division I. — Shell, subangular. 



Family 1. — Having a smooth surface. 



Spengleri — Spengler's. *Stultorum — Simpletons'. 



Carinata — Keeled. Grandis — Great. 



Maculata — Spotted. Achatina — Agate. 



Corallina — Banded. *Triangularis — Triayigular. 



Lactea — Milky. Minutissima — Mimite. 



*Cin€rea — Ashy. Donaciformis — Donax. 



