MACTRA. 417 



or less glossy, microscopically freckled, often iridescent, and 

 slightly streaked lengthwise towards the front margin : pallial 

 scar distinct, with a short and tongue-shaped sinus : muscular 

 scars irregularly triangular, deep, and of equal size. L. 1*4. 

 B. 1-6. 



Var. 1. truncata. Shell longer in proportion to its hreadth, 

 gibbous, and of a thicker texture ; sides more compressed or 

 truncate ; umbones prominent; hinge and teeth stronger. M. 

 truncata, Montagu, Test. Brit. Suppl. p. 34 ; F. & H. i. p. 354, 

 pi. xxiii. f. 1. 



Var. 2. conjunctiva. Shell much smaller, having the same 

 proportions as the variety truncata, and also rather ventricose, 

 but thin ; umbones not more prominent than usual. L. 0*6. 

 B. 0-85. 



Var. 3. elliptica. Shell invariably smaller than the typical 

 form, broader in proportion to its length in consequence of the 

 sides being more produced, and of a thinner texture ; umbones 

 not prominent. L. 0-75. B. 1-25. M. elliptica, Brown, 111. 

 Conch. G. B. & I. p. 108, pi. xli. f. 6 ; F. & H. i. p. 356, pi. xxii. 

 f. 3, and (animal) pi. L. f. 1. 



Habitat : In gravelly sand, at low tides and at the 

 depth usually of only a few fathoms, on every coast. 

 Var. 1, South of Devon and Cornwall, Tenby, Irish 

 coasts, Firth of Forth, Clyde district, Orkneys, and 

 Lerwick. Var. 2. Shetland (J. G. J.). Var. 3. Sandy 

 bays, at depths of from 10 to 100 fathoms : the dorsal 

 area is sometimes ribbed in the variety elliptica, as well 

 as in the typical form. This exceedingly variable spe- 

 cies occurs in all our newer tertiaries, from the last 

 raised beach to the Coralline Crag. The ordinary kind 

 and the variety truncata have been chiefly noticed as 

 littoral and in southern latitudes, their furthest limit 

 being Sicily, where the former is also fossil ; the only 

 northern locality that appears to be recorded is the 

 Scandinavian coast, on the authority of O. F. Miil- 

 ler. The variety elliptica has essentially a northern 

 range, from Iceland (Steenstrup) to Kullen in the 



t 5 



