MACTRA. 359 



Mactra stuUorum, Penn. Brit. Zool. ed. 4, vol, iv. p. 92, pi. 52, f. 42. 

 „ lactea, PoLi, Testae. Sicil.pl. 18, f. 13, 14. 



„ triangtda, Philippi, Moll. Sicil. vol. i. p. 11, and vol. ii. p. 10 

 (from specimen). — Hanl. Recent Shells, sup. pi. 10, f. 40 

 (copied from Poli). 

 „ crassatella, Lam. Anim. s. Vert. (ed. Desli.) vol. vi. p. 107 (variety). — 



Deless. Rec. Coquill. Lam. pi. 3, f. 6. 

 „ cuneata, Sow. Min. Conch, pi. IGO. f. 7. — Woodward, Gaol. Norf. C2. 

 f. 10. 



The M. suhtruncata is of a triangular or trigonally ovate 

 shape, very inequilateral, more or less solid, opaque, and 

 glossy ; it varies in convexity from scarcely ventricose to 

 almost tumid, in the majority of examples, however, it 

 is merely subventricose ; the valves are of an uniform 

 whitish hue, covered with a very thin cinereous epider- 

 mis, which has often a yellowish tinge ; their surface has 

 its dorsal areas closely and more or less strongly grooved 

 concentrically, the anterior sulci being continued so as 

 to traverse a greater or smaller portion of the shell. 

 There are two well-marked varieties : the one which has 

 these sulci occupying almost its entire superficies is more 

 ventricose and abbreviated in form ; the other, in which 

 they are confined to the front extremity and the vicinity 

 of the lower margin, so that the general surface is smooth, 

 is more produced, being trigonally ovate and compara- 

 tively compressed. The ventral margin is nearly straight 

 in the middle, and rises anteriorly; the declination of 

 both dorsal margins is very considerable, particularly of the 

 front one, which, as it recedes from the beaks, is more 

 or less arcuated ; there is a great tendency to straightness 

 in the hinder dorsal or rather posterior outline. The hinder 

 dorsal area is greatly flattened, the front one is defined 

 by an angulated line running to the ventral corner from 

 the apex of the shell, which results from a slight depression 

 of that area, whose lips, however, distinctly project. The 



