158 MACTRID^. 



thin, with the very thin, short lateral teeth closely approaching 

 them ; ligament separated from the cartilage-pit by a lamella ; 

 pallial sinus rounded. M. elegans, Sowb. (cix, 5, 6\ is the only 

 species. Harvella has been considered a genus, with the two 

 preceding groups as subgenera of it, but they are all essentially 

 Mactras. 



MACTRODESMA, Conrad, 1868. Shell subtriangular ; cartilage- 

 pit very large, ovate and projecting much beyond the lower 

 margin of the hinge-plate ; anterior hinge-margin in the right 

 valve thick and continued much beyond the beak ; hinge of left 

 valve with a profoundly elevated A-shaped cardinal tooth, con- 

 nected with the hinge-line above it only at the base of the tooth ; 

 lateral teeth short, thick, subequal ; pallial sinus narrower and 

 deeper than in Mactra, ending in a line opposite to the middle 

 of the cartilage-pit ; muscular scars very large. M. ponderosa, 

 Conr. Miocene ; Maryland. 



PsEUDOCARDiUM, Gabb, 1866. 



Etym. — Pseudo^ false, and cardium, a generic name. 



Diatr. — Cardium Gabbi, Remond. Miocene and Pliocene; 

 California. 



Shell thick, heav}', resembling Lpevicardiuvi externally; liga- 

 ment internal ; lunule cordate ; left valve with a large cartilage- 

 pit and a A-shaped tooth, which articulates in a corresponding 

 djBpression in the right valve ; two lateral teeth in each valve, 

 very strong and prominent. 



Rangia, Desmoulins, 1832. 



Syn. — Gnathodon, Rang, 1834. Clathrodon, Conr., 183'7. 



Bistr. — 1 sp. New Orleans. (3 other sp.? Mazatlan, Cali- 

 fornia, Moreton B., Australia. Petit.) Fossil, 3 sp. Cret. — ; 

 Petersburg, Virginia. 



Shell oval, ventricose ; valves thick, smooth, eroded ; epidermis 

 olive ; cartilage-pit central ; hinge-teeth f ; laterals doubled in 

 the right valve, elongated, striated transversely^ ; pallial sinus 

 moderate. 



Animal with the mantle freely open in front ; margins plain ; 

 siphons short, partly united ; foot very thick, tongue-shaped, 

 pointed ; gills unequal, the outer short and narrow ; palpi large, 

 triangular, pointed. 



jR.cyrenoides, Desm. (cix, T), was formerly eaten by the 

 Indians. At Mobile, on the Gulf of Mexico, it is found in colo- 

 nies along with Cyrena Garolinensis, burrowing two inches deep 

 in banks of mud ; the water is only brackish, though there is a 

 tide of three feet. Banks of dead shells, three or four feet thick, 

 are found twenty miles inland. Mobile is built on one of these 



ii- banks. The road from New Orleans to Lake Ponchartrain 



