BIVALVES. MACTHA. 21 



MACTRA.— -Kneading-trough. 



Animal-— ct Tethys: Shell bivalve, unequal sided, iqid- 

 valve; middle tooth of the hin</e complicated, with a 

 small hollow on each side; lateral ones remote, and in- 

 serted into each other. 



The genus Mactra has little to boast of, either in regard 

 to beauty or variety. The number of species amounts 

 but to twenty-seven, and in those no great difference of 

 coloring or form is manifested. 



The shape of the Mactra is usually inclined to be some- 

 what triangular; ho^vever, in some instances, it is more 

 oblong. 



The surface of the exterior is generally smooth, or mi- 

 nutely striate ; some exceptions, however, may be found, 

 as in the Mactra Plicataria, and others, which exhibit a 

 wrinkled or ribbed appearance, similar to that observed 

 in the different species of the Cardium, but in a reverse 

 direction ; they being ribbed or plaited longitudinally, 

 whereas, in the Mactra, the elevations and their adjacent 

 grooves are placed transversely. 



The generality of Mactrse are of a delicate construc- 

 tion, and afford a semipellucid or half-transparent ap- 

 pearance. They are, for the most part, thin, brittle, and 

 remarkably light; and in some species the valves are 

 open or gaping. 



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. 



