181 



Venus divaricata, Linn, illustrates this genus, of which I do not know 

 that any fossil specimens are known. 



GIX. Lutraria. A transverse inequilateral shell, gaping at the extre- 

 mities ; two oblique and diverging hinge-teeth accompanying a large pit 

 for the cartilage. No lateral teeth. 



Lutraria elliptica, List, conch, t. 415. /. 259, is taken by Lamarck to 

 illustrate this genus, which is, I believe, unknown. 



CX. Mactra. An equivalved, inequilateral, transvere bivalve, a lit- 

 tle 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 mactrse are marine shells, and do not appear to have been fre- 

 quently 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. solida, 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. Erycina. 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, 



