of Mactraceae, only by the character of the internal liga- 

 ment, seems to be questionable. 



Cuculiaea corresponds with the present genus in its 

 general appearance, and we know of no other difference 

 than that exhibited by the teeth, which are less regular, 

 and those at each extremity of the series are transversely 

 elongated. 



Pectunculus is a closely related genus, differing chief!}' 

 by the arquated series of teeth ; the general orbicular form, 

 and by the valves closing completely so as to prevent the 

 passage of a byssus. 



Ferussac in his Tabl. Syst. places Trigonia in this fami- 

 ly ; but the teeth are dissimilar, and the animal is un- 

 known. 



The Arcae are marine shells, and, as Lamarck observes, 

 some of the species have one of the valves extending be- 

 yond the other on the basal edge. Some are covered by 

 a kind of hairy epidermis. Blainville says, that the ani- 

 mal has the body thick, of a somewhat variable form : 

 abdomen provided with a pedunculated, compressed foot, 

 longitudinally divided; mantle with a simple range of 

 cirri, and a little elongated before ; the tentacula are very 

 small and very slender. 



ARCA STAMINEA. 



DESCRIPTION. 



Shell thick, prominently convex ; with about twenty- 

 eight ribs which are rounded and narrower than the inter- 



