38 BIVALVES. MACTRA, 



The generality of Maclrse are of a delicate construc- 

 tion, and have a semipellucid appearance. They are, for 

 the most part, thin, brittle, and remarkably light. 



The most prevailing color is blueish or yellowish- white, 

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

 purple, heightened with rich tints of the same color : 

 others, again, are of a brilliant lilac, passing into a deli- 

 cate blue. 



The hinge of the Mactra is its best distinction from all 

 the other genera of Bivalves^ for the middle tooth is al- 

 most invariably complicated, and of a triangular form, 

 having a small hollow on each side; the lateral teeth are 

 remote from the beaks and inserted into each other. The 

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

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

 thinner than paper. 



The Mactrse are mostly equivalves; in a few speci- 

 mens 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. — Kneading-trough. 



Spengleri — Speugler's. Glabrata — Smooth, 



Plicataria — Plaited. Nitida — Delicate. 



Papyracea — Paper. Corallina — Banded. 



Striatula — Suhstriated, Lactea — Milky. 



Striata — Striated. *Stultoriim — Simpletons'. 



Rotunda ta — Roundish. Grandis — Great. 



