372 MACTRID^. 



hinder dorsal edge, which is the more elevated one, is 

 almost straight, and declines in but a very trifling degree. 

 The umbones are not very projecting ; the beaks, which 

 are incurved and acute, lean scarcely, if at all, to either 

 side. There is no umbonal ridge, nor the slightest appear- 

 ance of a distinct lunule. The internal surface is of a 

 glossy bluish white ; in the left valve, in advance of the 

 large curved triangular cartilage-pit, is a solid truncated 

 reversed V-shaped primary tooth (which does not reach 

 the basal line of the hinge-margin), preceded by a very 

 thin lamellar sub-pyramidal one : in the right valve is only 

 the very thin and fragile anterior wall of the cartilage-pit, 

 which resembles a tooth, and a rather curved slanting an- 

 terior laminar one, which is considerably elevated at its 

 lower extremity, and acutely pointed. 



The largest of our specimens (from the Welsh coast) 

 measures five inches and a half from side to side, and three 

 inches from the umbones to the opposite margin. These 

 proportions are not universal, as the breadth of another 

 example of four inches in length is only two inches. 



The animal is sub-cylindrical, with a siphonal tube fully 

 as long as the body, or longer. The mantle is closed pos- 

 teriorly and frontally ; anteriorly there is rather a large 

 opening for the passage of the thick, long, white foot, 

 which does not appear to be furnished with a byssal 

 groove. The siphons are united almost to their extremi- 

 ties, which are but very slightly separated from each other. 

 The tube is white, thick, and corrugated at the base, 

 thinner and yellow, speckled with brown towards the ex- 

 tremity. " The branchial tube,'' according to Mr. Clark, 

 " is clotted with about ten yellow rays, dotted with mi- 

 nute points, and each ray more or less ciliated on one or 

 both sides ; the anal tube turns upwards, and has around 



