MACTRA. 415 



Genus II. MACTRA*, Linne. PL VIII. f. 2. 



Body oval, tumid : mantle thick, open in front : tubes united, 

 enclosed in a sheath, of equal length, and extensile ; orifices 

 ciliated. 



Shell triangularly oval, ventricose, nearly equilateral, 

 slightly gaping at the posterior end, concentrically striated, 

 and occasionally (but slightly) marked -with longitudinal lines : 

 beaks turned towards the anterior side, separate : teeth, two 

 thin, erect, and diverging cardinals in each valve ; those in the 

 right valve are of unequal size ; and those in the left valve 

 equal-sized, united at their bases, and apparently forming a 

 single forked or chevron-shaped tooth ; laterals large and 

 laminar, two on each side in the right valve, and one on each 

 side in the left valve : pallia! scar having a short sinus. 



The shells comprised in this ancient genus are always 

 recognizable by their triangular shape and strong hinge- 

 process. Searles Wood says that, in a fossil state, they 

 have been obtained in the Lias and Greensand formations. 

 The genus is largely represented in the newer Tertiaries, 

 and continues in a nourishing condition at the present 

 day. They seem to prefer sand to mud as a dwelling- 

 place. 



A. Shell of a plain or uniform hue ; lateral teeth and sockets 

 perpendicularly striated. — Spisida, Gray. 



1. Mactra so'LiDA-fv, Linne'. 



M. solida, Linn. Syst. Nat. p. 1126 ; F. & H. i. p. 351, pi. xxii. f. 1, 5, and 

 (siphons) pi. L. f. 2. 



Body more compressed than in most of its congeners, milk- 

 white, yellow, or light orange, occasionally tinted with brown : 

 mantle less fringed on the posterior side than in other parts ; 

 edges thickened : tubes short, but often extended three quar- 

 ters of an inch in specimens considerably under the average 

 size ; the orifice of each is furnished with 12-20 short pale- 

 yellow, brown, or reddish cirri, which spring from bulbs ; 



* A kneading-trough. t Solid. 



