280 GINGLYMACONCHA. AMPHIDESMADZ. 
taken by Mr. Cranch, who observed it likewise in the Plymouth 
Sound. The only living specimen that I have seen was taken 
up by the trawl in Torbay, and is in Mr. Loscombe’s cabinet. 
From the examination of this last specimen in a recent state, 
I was enabled to discover its proper situation in the natural 
system. 
Genus 28. OrIxa. 
Shells when viewed laterally are subtriangular, longer than 
broad, the inferior margin rounded; umbones nearly central. 
Hinge in the left valve with a lamella before and behind the 
umbones ; the processes bearing the ligament vertical and ex- 
cavated ; the left valve with a tooth before the ligament, which 
is received between two teeth, similarly situated in the nght 
valve. 
ORIXA TENUIS. 
Shell white, thin, slightly opake, finely striated concentrically. 
Length five-sixteenths ; height four-sixteenths of an inch. 
Mactra tenuis, Mont. Test. Brit. 572. t. xvu. f. 7. figura 
pessima ; M. § R. Trans. Linn. Soc. viii. 72; Dill. Dese. 
Cat. 142; Turt. Conch. Dict. 84. 
Ligula tenuis, Mont. Test. Brit. Supp. 23; Flem. Edinb. En- 
cycl, vil. 88. 
Amphidesma tenuis, Lam. Hist. Nat. des Anim. sans Vert. v. 
492. 
Inhabits the shores of estuaries, but is of rather rare occur- 
rence. It has been taken at Southampton and near Weymouth 
by Montagu and Mr. Boyer, and at Portsea by Mr. Holloway. 
Genus 29. TRIGONELLA. 
Trigonella, Da Costa. 
Shell longer than high ; when viewed on the side, somewhat 
triangulate, the lower margin rounded; umbones nearly central. 
Hinge without lamellee ; the processes bearing the cartilage 
produced vertically and excavated ; the left valve with an ob- 
