340 . Fossil and Recent Shells. 



MACTRA. 



1. J\l. clathrodonta. Shell subovale; posterior side cuneate ; 

 dorsal slope but little curved ; posterior lateral tooth much elongated 

 and elegantly striated ; fosset oblique, ovate ; anterior lateral tooth 

 elongated. Length, two inches. 



Locality. — ^Yorktown, Va. Upper marine formation. 



So closely does this shell resenable Clathrodon cuneata* 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 plaae 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 Solecurtvs Caribceus 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 showed me 

 others from the coast of South America. 



2. JVI. congesta. Shell triangular, convex, thick ; posterior side 

 cuneate, beaks nearJy central ; lunule none ; fosset small, circular, 

 profound ; lateral teeth thick. Length, one inch. 



Locality. — Suffolk, Va., where it is extremely abundant. A much 

 smaller variety occurs at James river, generally shorter in proportion 

 to the height, and with central beaks. Upper marine formation. 



3. M. confraga. Shell subtriangular ; narrow, somewhat thick, 

 with coarse concentric lines; umbo oblique; beaks a little elevated, 

 approximate ; posterior side longer and less obtuse than the anterior; 

 fosset large cordate, oblique; lateral teeth strong; muscular im- 

 pressions large. Length, two inches. 



Resembles M. solidissima, Chem., but the apex is not directed 

 forwards as in that species. It was found by Mr. Finch in Maryland 

 and is from the upper marine beds. 



4. M. modicella. Shell subtriangular, compressed ; posterior 

 side shortest and abrupt or truncated at the extremity ; fosset a little 

 oblique, triangular ; lateral teeth strong. Length, three fourths of 

 an inch. 



Locality. — Yorktowh, Va. Upper marine. 



* Hangia cyrenoides, M. des Mouliiis. 



