340 Fossil and Recent Shells. 
MACTRA. 
1. M. clathrodonta. Shell subovate; posterior side cuneate; 
dorsal slope but little curved; posterior lateral tooth much elongated _ 
and elegantly striated ; Sanat oblique, ovate; anterior lateral rion 
elongated. Length, two inches. 
Locality.—Yorktown, Va. Upper marine formation 
So closely does this shell resemble Clathrodon cine Gray, 
that on a. superficial examination it might almost be taken for the 
same species; the hinge, however, is that of a true Mactra. The 
Clathrodon has been arranged among fresh water shells, and con- 
‘sidered as nearly allied to Cyrena, but I should prefer to place it 
next to Mactra in a natural arrangement, as the hinge does not_ 
greatly differ from that genus, and the palleal impression is exactly 
similar. With regard to its habit of living in brackish water, it may 
be observed that other marine shells are frequently found in like sit- 
uations, and I have seen the Solecurtus Caribeus and other shells of 
our coast many miles above the mouth of the Potomac and in its: 
tributary rivers. I obtained many specimens of Clathrodon from 
the sea beach near Cape Henry, Va., and Mr. I. Lea outs me 
others from the coast of South America. aan 
2. M. congesta. Shell triangular, convex, inks : en 
cuneate, beaks nearly central; Junule none ; fosset ‘anil circular, 
profound ; lateral teeth thick. Length, one inch. 
Locality.- , Va., where it is extremely abundant. A much 
smaller variety occurs at James river, generally shorter in proportion 
to 28 a and with central beaks. Upper marine formation. 
M. confraga. Shell subtriangular ; narrow, somewhat thick, : 
x coarse concentric lines; umbo oblique; beaks a little elevated, 
approximate ; posterior side Siew and less obtuse than the anterior; 
fosset large cordate, oblique; lateral teeth strong; muscular § im- 
pressions large. Length, two inches. 2 gprs 
Resembles MM. solidissima, Chem., but the apex is not ‘ irecte ted 
. forwards as in that species. It was cine by Mr. Finch in Ma 
ERs sep merine bots 1 aes 
- M. modicella. Shell subtriangular, Scchgreneds posterior 
side shortest and abrupt or truncated at the extremity ; fosset a little 
oblique, triangular ; lateral teeth strong. Length, three fourths of 
Locality —Yorkiown, Va. leper netinhss | aegis 
> nengit scunaiien M. des. Meolian, 
