ISOCARDIA. La Marck. 
Gen. Char. An equivalved cordiforai bivalve, with 
distant involute beaks ; teeth of the hinge two, 
compressed, one of them curved under the 
beak ; one lateral tooth extended beneath the 
lunette; ligament external, anteriorly furcated. 
In several positions the shells of this genus present a 
cordiform outline, because they are generally very gib- 
hose, and the notch in which the hinge is imbedded is 
very large ; the incurved beaks assist greatly in distin- 
guishing them when the hinge teeth cannot be examined, 
as is often the case in fossils. Ohama Cor of Linn e us 
is the type of the genus. 
ISOCARDIA minima* 
TAB. CCXC V'—Fig, L 
Spec. Char. Globose, subdeltoid ; anteriorly 
slightly truncated, posteriorly flattened; cordate. 
The flat heartshaped posterior side of this shell distin- 
guishes it from Isocardia Cor which other ways it re- 
sembles except in size, being much smaller, the anterior 
side is sharply produced a little way and slightly trun- 
cated. 
I have only met with casts of this shell which do not 
exhibit the hinge teeth, and are besides rather rugged, 
but no better are likely to be found. I am indebted to 
Mr. Salmon for them ; they were collected in Wiltshire, 
probably in the Co nib rash Limestone. I have similar 
ones from Madagascar. 
ISOCARDIA tener. 
TAB. CCXCV.— 1%, 2. 
Spec. Char. Ob ovate, with produced beaks; 
anteriorly subtruncated, posteriorly rounded ; 
shell very thin, smooth. 
An elegant thin shell, rather wider than long, with a 
straightish front; it is less gibhose than the last, and the 
