MACTRA. 77 



localities ; but I apprehend that M. falcata, Gould, is the species 

 retbrred to. 



This shell is inferior in size to the 31. soUdissima only. The 

 largest valve I have seen measures four and one fifth by two and 

 four fifths inches. In general it is found smaller than the dimen- 

 sions given. Such specimens have a straw-colored epidermis. 



Its size, shape, and surface distinguish it from M. grandis, 

 Chemn., 31. solida, and all other known species except 31. soUdis- 

 sima. Compared with that more common shell, it differs in its 

 coarser surface, its thick, dusky epidermis, its less elevated beaks 

 and less convex valves, the longer and less elevated anterior por- 

 tions, the rough, oval portion behind the beaks embracing more 

 of the shell, the more feeble hinge, the stout and firm V tooth, 

 and the deep sinus of the pallial impression. 



I have very little doubt that this is the shell described by De- 

 shayes as 31. grandis. The description corresponds well. But, 

 as it is not figured or quoted by other authors, and the habitat 

 was not known to him, it is impossible to decide with certainty. 

 Singularly enough too, Deshayes himself does not cite it in his 

 new edition of Lamarck. Provided it be the same, however, the 

 specific name must necessarily be changed, as his name was pre- 

 viously given to a different species by Chemnitz. 



This appears to be the same species, a small specimen of which 

 is figured by Mr. Gray in the Appendix to Beechey's Voyage, 

 under the name of 31. similis. This name is preoccupied by Mr. 

 Say. 



It is objected that the name ovalis has been used for a fossil 

 species ; but if the generic term Spisnla be adopted, this shell will 

 retain the above name. As before observed, Philippi's ponderosa 

 appears to me to represent a variety of 31. soUdissima, rather than 

 this species. 



Mactra lateralis. 



Tigs. 34, 35. 



Shell small, triangular, very convex, nearly equipartite ; beaks elevated, the 

 spaces before and behind them broad heart-shaped. 



Mactra lateralis, Say, Jonrn. Ac. Nat. Sc. ii. 309. — Covrad, Amer. Mar. ronch. 62, 

 pi. 14, %. 4. — De Kay, Nat. Hist. New York, 230, pi. 29, fig. 287. — Philippi, 

 Abbild. pi. 1, fig. 3. 



Shell small, triangular, tumid, nearly smooth, shining ; nearly 

 equipartite, the posterior part somewhat prolonged, and sloping 



