59 



a fosset for the cartilage situated obliquely beneath it; 

 lateral teeth two, compressed, approximated to the 

 fosset, double in the left valve ; ligament partly ex- 

 terior ; palleal impression with a sinus, not very pro- 

 found. 



OBSERVATIONS. 



This genus embraces a great number of recent as 

 well as fossil species, and is related to Amphidesmap 

 Lutrarittf Erycina, Rangia and Mesodesma. 



The species burrow a few inches deep in the sand, 

 and generally inhabit the ocean 5 I have seen, how- 

 ever, a delicate Mactra in the cabinet of Dr. Rus- 

 chenberger, U. S. N., which he procured alive in 

 fresh water in Tombez river, Peru ; the species, in 

 this respect, having the peculiar habits of the Rangia. 



Thirty-four recent species are known ; and in the 

 Tertiary strata of Europe, Deshayes has noticed four- 

 teen ; six occur in the Pliocene strata of Virginia and 

 Maryland, and three in the Eocene of Alabama. 



MACTRA DEAURATA. 



Tab. XIV, /^. 1. 



SPECIFIC CHARACTER. 



Shell subovate, solid, compressed ; posterior side 

 short, truncated, and somewhat angular; anterior 



