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 Modiola, which it seems to have nearly lost in the 

 adult, where the umbo has become more pointed, like that of Mytilus. 



ARCA. Linnaus. 



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 striae ; 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, exhibiting 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 Ciicuttaa, 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 



