78 EOCENE MOLLUSCA. 
are two rather unequal and not very perfect teeth immediately beneath the umbo, within 
which is also the impression of the oral adductor; the external radiations are numerous 
and flat, and they bifurcate at an early age; the interspaces are ornamented with raised 
lines of growth, which impart to them an irregularly cancellated appearance, and these, if 
they exist, are not represented in the French shell. In the young state, the form 
resembles more the normal state of Afodio/a, which it seems to have nearly lost in the 
adult, where the umbo has become more pointed, like that of Mytilus. 
ARCA. Linnaeus. 
Generic Character. Shell inequilateral, generally equivalved, more or less quadrate 
or trapezoidal; ventral margin sometimes closed, at others open or sinuated ; externally 
covered with radiating striz ; umbones distant, with more or less open area for connexus ; 
hinge straight, with many teeth; palleal impression entire. 
This is almost exclusively a marine genus, and comprehends nearly five hundred species. 
Some of these, however, vary so materially in the form, number, and arrangement of the 
denticles upon the straight and elongated margin of the hinge, as to have been separated 
into several genera or sections, in accordance with those variations. ‘The generality of 
species show an opening more or less in the ventral margin, indicating a habit in the 
genus to spin a byssus. 
In some few species there is an inequality in the valves; when this is the case, the left 
one is the larger of the two, and this inequality is found principally in those species 
which are without a sinuated margin. 
The hinge, or dental area, is quite straight; this in some species is furnished with nu- 
merous small teeth placed at right angles to the line of it; in others, the denticles are few in 
number and are variously inclined, until they become at the extremities parallel with the 
hinge-margin, exhibitmg every possible degree of intermediate variation. ‘The shells that 
have been generally included in this genus from the older rocks have most of them very 
oblique denticles, like those of Cvcud/ea, but they are not restricted to that form of dentition. 
The area between the umbo and the dental margin over which the connector is spread is 
at times very large and open; the diverging and chevron-formed lines which ornament 
this space are deeply impressed in the shell; into them a portion of the ligament has 
been inserted for strength and protection, as also to have an intervening raised portion 
on which to act as a fulcrum. There is in this character an approach to Limopsis, 
in which there is an angular depression; but it has not any analogy with the bipartite or 
amphidesmous form of connexus, inasmuch as the action of the whole connector is liga- 
mental, acting by contraction and elongation. In Pectunculus the area is marked with a 
single divergence, forming an obtuse angle; but in the present genus, in which some of 
