158 MACTRIDA. 
thin, with the very thin, short lateral teeth closely approaching 
them ; ligament separated from the cartilage-pit by a lamella ; 
pallial sinus rounded. J. 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. MU. ponderosa, 
Conr. Miocene; Maryland. 
PsEUDOCARDIUM, Gabb, 1866. 
EHtym.—Pseudo, false, and cardium, a generic name. 
Distr.—Cardium Gabbi, Remond. Miocene and Pliocene; 
California. 
Shell thick, heavy, resembling Levicardium externally ; liga- 
ment internal ; lunule cordate ; left valve with a large cartilage- 
pit and a A-shaped tooth, which articulates in a corresponding 
depression in the right valve ; two lateral teeth in each valve, 
very strong and prominent. 
RanerA, Desmoulins, 1832. 
Syn.—Gnathodon, Rang, 1834. Clathrodon, Conr., 1837. 
Distr.—l1 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 2 an 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. 
R. cyrenoides, Desm. (cix, 7), was formerly eaten by the 
Indians. At Mobile, on the Gulf of Mexico, it is found in colo- 
nies along with Cyrena Carolinensis, 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 
Il-banks. The road from New Orleans to Lake Ponchartrain 
