354 MACTRID^. 



Duncansby Head (M'Andrew). It delights in estuaries, 

 though some of the largest varieties occur in localities away 

 from the neighbourhood of fresh-water, as on the coasts of 

 Lewis. It ranges throughout the European seas, and has 

 inhabited them ever since the epoch of the coralline crag. 



M. TRUNCATA, MoutagU. 



S'olid, rounded, triangular, subequilateral ; umbones oblique 

 and very prominent; dorsal areas and lateral teeth grooved. 



Plate XXIII. %. 1. 



Lister, Hist. Conch, pi. 253, f. S7. 

 Mactra truncata, Mont, (not of Turt. Dithyra Brit.) Test. Brit. Supplement p. 34. 

 — Turt. Conch. Diction, p. 81.— Flem. Brit. Anim. p. 4'27. 

 — Brit. Marine Conch, p. 46. — Dillw. Recent Shells, vol. i. 

 p. 140. — Hanl. Recent Shells, p. 32, sup. pi. 9, f. 1. 

 „ sithtruncata, DoNOV. Brit. Shells, vol. iv. pi. 126. 

 „ crassa, Turt. Dithyra Brit. pp. 6.0, 258, pi. 5, f. 7- 



The Mactra truncata of Montagu is most closely allied 

 to that ancient species the M. solida ; but although requi- 

 ring the strictest scrutiny to discriminate it from certain 

 forms of that shell, nevertheless appears, throughout the 

 long series of specimens in all stages of growth and col- 

 lected in various localities which we have examined, to 

 preserve its peculiar distinctive characters. The contour is 

 rounded-triangular ; the texture thick, solid, opaque, and 

 of an uniform dirty white ; the surface dull or but very 

 moderately glossy, and more or less evidently marked, in 

 a concentric direction, with regular rather broad striae, 

 which, however, very readily become wholly or partially 

 obsolete. The ventral margin is irregularly arcuated in 

 the middle, and both dorsal edges meet it in a single unin- 

 terrupted rapid slope, whence arises the trigonal outline of 

 the shell. The valves are ventricose and subequilateral, 



