Eocene Fossils of the United States. 397 
Upper Marlborough, Prince George Co., Maryland. 
A single imperfect valve is all I found of this species. It is 
remarkably flat, particularly over the umbo. It was imbedded 
in a hard rock or indurated marl, and is composed of crystallized 
carbonate of lime. 
AMPHIDESMA. 
The oldest known species of Amphidesma of Lamarck, oceur 
in the Eocene. Only one fossil species was known to Deshayes 
when he published his table in 1833. In the American Eocene 
I find three species; in the Miocene, eight; and two recent ones 
occur on the Keys of Tampa bay, Florida. 
Deshayes supposed this to be an artificial genus as grouped by 
Lamarck ; but of all the bivalve genera, we know none more natu- 
ral and easily recognized, considered in reference to American spe- 
cies ; although in some there are lateral teeth, in others none, and. 
some species have them in one_ valve only. The most striking 
character is the very oblique, elliptical or oblong fosset. Perhaps 
the genus might rank next to Mi esodesma in the family Mactride. 
AmpuipesMa Linosa. Ovate, plano-convex with fine regular 
concentric prominent lines; right valve with a somewhat promi- 
nent obtuse fold over the umbonial slope, the base of which is 
emarginate ; posterior side short ; extremity subtruncated or very 
obtusely rounded, direct ; beaks slightly prominent ; fosset pro- 
duced, elliptical. (Plate IV, fig. 2.) 
A. linosa, Con.; Foss. Shells of Tert. Form., p. 42, 
Claiborne, Alabama. 
I have only two valves of this species, which much resembles 
a Tellina exteriorly. The palleal sinus is very profound. In 
the right valve are two diverging compressed cardinal teeth, and 
the lateral teeth are very distinct. 
AmPHIDESMA TELLINULA.. Subovate or subtriangular, thin, ven- 
‘tricose ; dorsal margins very oblique, the posterior one rectilinear ; 
posterior side narrower and rather shorter than the anterior, ex- 
tremity truncated and very much above the line of the base ; 
basal margin profoundly rounded ; cardinal tooth double ; fosset 
small; lateral teeth obsolete. (Plate IV, fig. 5.) 
Claiborne, Alabama. 
Of this species I have but one left valve. The fosset of the 
hinge is shorter than is usual in the genus, and much less oblique 
Sxconp Sznres, Vol. I, No. 3.—May, 1846. 1 
