MACTRA. 75 



at Newport, I presume to be the M. similis of Say. Whether it be 

 simply a variety of M. solidissima or not, I will not presume to de- 

 cide from this one specimen ; and as it is not strictly a Massachu- 

 setts shell, I shall merely notice it in this way. The shell has cer- 

 tainly quite a different aspect. It is triangular, the beaks are more 

 elevated, the marginal outlines are straight, and the comparative 

 dimensions vary as follows : — 



M. solidissima, length, one and four fifths inches; height, one 

 and three twentieths inches ; breadth, three fifths of an inch. 



M. similis, length, one and four fifths inches; height one and 

 four tenths inches ; breadth, four fifths of an inch. 



[N(j doul)t is now entertained of their distinctness.] 



M. pondcrosa of Philippi I should regard as a form of this sjie- 

 cies and not of the following. 



Dr. Stimpson found in aliundance a most extraordinary form, 

 short and ventricose, but with the striated teeth and general char- 

 acters of the species. Its relative dimensions were, length, three 

 and one eighth inches ; height, two and one half inches ; l)readth, 

 one and one half inches ; and these proportions are maintained 

 through the very young specimens. It inhabits Grand Maiian, and 

 may be called var. curta. 



Our species all belong to that sultdivision of the old Linnean 

 genus which Dr. Gray calls Spisula. 



I have received a shell, about three fourths of an inch long, 

 from Dr. Loven of Stockholm, which he calls M. lutcola. I can- 

 not perceive that it differs from the young of this shell, of a cor- 

 responding size. 



Mactra ovalis. 



Fig. 32. 



Shell large, thick, obovate, coarse, nearly equipartite, covered with a tough, 

 dusky-brown epidermis; V tooth strong-; lateral teeth not striated; sinus of pal- 

 lial impression deep. 



Mactra similh, Gray, Append, to Becchey's Voyage, pi. 44, fig. 8. 



Mactra grandls, Desh.wes, Encyc. Meth. Vers, ii. 395, not ]\[. ijrandis of Chemnitz and 



others. — Mighels, Journ. Bost. Soc. Nat. Hist. iv. 316. 

 Mactra ovalis, Gould, Inv. Mass. 1st cd. 53. -De Kaa', Nat. Hist. New York, 230. — 



MiDDENDORFF, Siber. Reise (Moll.) 103. — Reeve, Conch. Icon. fig. 36. 

 Mactra pondcrosa, Stimpson, Shells of New England, 20. 

 Mactra polijmjma, Stimpson, Check List, East Coast Shells, 3. 



Shell large, thick and coarse, somewhat compressed, sub-oval, a 

 little shortest anterior to the beaks, and, the anterior slope of the 



