MACTRA. 419 



and southern variety, and M. elliptica as the deep-water 

 and northern variety of one and the same species. Clark 

 says that M. elliptica appears distinct, because of its 

 delicacy and elegance ; but he adds that M. solida is at 

 Exmouth strictly a littoral species, and that the other is 

 never taken except by the dredge in the coralline zone, 

 six miles from the shore. Every conceivable gradation 

 of shape and solidity may be seen in a recent state ; 

 and the union of M. solida and M. elliptica is cemented 

 by paheontological researches — showing the advantage 

 (if not the necessity) of such investigations in the study 

 of the Mollusca. As Searles Wood has remarked, spe- 

 cimens of M. ovalis from the Red Crag may belong to 

 either species. I may also observe that when M. solida 

 gradually finds its way into deeper water than it had 

 been accustomed to — for instance, in descending from 

 the Dogger bank down its slopes to the Silver-pits on 

 the Yorkshire coast — the shell becomes more slender 

 and glossy, although of nearly the usual size. It has 

 then all the appearance of a large M. elliptica. 



The typical form is the Trigonella gallina of Da Costa, 

 M. vulgaris of Chemnitz, and probably M. castanea of 

 Lamarck; and the variety truncata is the Trigonella 

 zonaria of Da Costa, M. subtruncata of Donovan (but 

 not of Da Costa), M. crassatella of Lamarck, and M. 

 crassa of Turton. The latter variety was figured by 

 Lister as " Concha crassa." 



2. M. subtrunca'ta *, Da Costa. 



Trigonella subtruncata, Da Costa, Brit. Conch, p. 198. M. subtruncata, 

 R & H. i. p. 358, pi. xxi. f. 8, xxii. f. 2, and (siphons) pi. L. f. 3. 



Body convex, milk-white with sometimes a faint tinge of 

 yellow : mantle having its edges crenated or serrated rather 



* Somewhat truncated or lopped. 



