181 
Venus divaricata, Linn, illustrates this genus, of which I do not 
know that any fossil specimens are known. 
CIX. Lutraria. A transverse inequilateral shell, gaping at the 
extremities ; two oblique and diverging hinge-teeth accompanying a 
large pit for the cartilage. No lateral teeth. 
Lutraria elliptica, List, conch. Tab. 415, Fig. 259, is taken by 
Lamarck to illustrate this genus, which is, I believe, unknown. 
CX. Mactra. An equivalved, inequilateral, transverse bivalve, a 
little gaping at the sides ; the hinge-tooth complicated, with an 
adjacent little pit ; the lateral teeth rather remote, compressed, and 
inserted : cartilage internal, inserted in the pit of the hinge. 
The mactra; are marine shells, and do not appear to have been 
frequently found fossil. Lamarck describes but one species, M. 
semisulcata, as found fossil in the neighbourhood of Paris. The only 
English fossil-shell of this genus, with which I am acquainted, is one 
which is found in the Essex bank of fossil shells. 
Two circumstances served to give some degree of ambiguity to 
this shell : the great width between its strongly projecting transverse 
ribs, and the structure of its lateral teeth ; which, as in those of the 
genus Trigonia, were transversely sulcated : the two flat diverging 
teeth of the left valve on their outsides, and the flat receiving teeth 
on their inside. But Mr. Pennant, British Zoology, No. 43, A, 
observes, that in dead shells of M. sotida, the striae appear like high 
ribs ; and I was surprised to find, on examining the teeth of the 
recent shells of this species, that a similar structure with that observed 
in the teeth of the fossil specimen is perceptible — a circumstance 
which has not been generally noticed. 
CXI. Erycinci. An equivalved, inequilateral, transverse bivalve. 
The hinge-teeth, two, diverging upwards, with a small intermediate 
pit ; the lateral teeth compressed and oblong. The cartilage inserted 
in the hinge -pit. 
The cartilage of these shells is inserted interiorly, as in the mactrae, 
