BIVALVES. CARDIUM. 37 



Latum. — Broad. Amboinense. — Amhoyna. 



*Pygmseurn. — Small. Squamosum. — Scaly. 



Maculatum. — Spotted. Cancellatum. — Cancellated. 



Flexuosum. — Flexuoiis. Rubiginosum. — Rusty. 



*Fluviatile. — River. Albidum. — Whitish. 



Gaditanum. — Cadiz. Virescens. — Greenish. 



Brasilense. — Brasil. Fasciatum. — Banded, 



MACTRA. — Kneading-trough. 



Animal — a Tethys: Shell bivalve, unequal sided, equi- 

 valve ; middle tooth of the hinge 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 

 only to twenty-seven, and in those no great diflference of 

 coloring or form is observable. 



The Mactra is usually of a triangular form ; but, in some 

 instances, it is rather oblong. 



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

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

 as the M. plicataria, and others, which exhibit a wrin- 

 kled or ribbed appearance, similar to that observed in 

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

 rection ; the latter being ridged or plaited longitudinally, 

 while in the Mactra, the elevations and their adjacent 

 grooves are placed transversely. 



