BIVALVES MACTRA, 



35 



Oblongum. 



Crassum. 



Latum. 



*PygiiiaBum. 



Maculatum. 



Flexuosum. 



*FJuviatile. 



Gaditanum. 



Brasilense. 



Amboinense. 



Squamosum. 



Cancellatum. 



Rubiginosum. 



Albidum. 



Virescens. 



Fasciatam. 



MACTRA.— -Kneading-trough. 



Animal'^a Tethys : Shell bivalvCf unequal sidedt eqni- 

 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 

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

 coloring or form is manifested. 



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

 what triangular ; however, 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, 

 es 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 

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