MACTRA. 357 



the anterior, becomes somewhat rectihncar in the adult, 

 and its declination is similarly moderate. The beaks are 

 very nearly central, very depressed, acute at their apices, and 

 almost directly inflected, projecting, however, very slightly 

 forward. The anterior side is, if either, a little the short- 

 er, and is somewhat rounded, but not symmetrically, at its 

 extremity; the posterior termination is subangulated be- 

 low in the adult, but more rounded when immature. 

 The hinge occupies four-ninths of the entire length of the 

 valves ; the cartilage-pit is triangular, oblique, and rectan- 

 gular in front at the base ; the primary teeth occupy nearly 

 the entire breadth of the margin, and have the lobes nearly 

 equal in the left valve. The lateral lamina? are nearly equally 

 approximate, elongated, and very solid, rather flat-topped, 

 and most delicately grooved perpendicularly ; these in solida 

 are thicker, and less closely but more strongly sulcated. 



The largest example we have ever seen was an inch 

 and a half long, and four-fifths of an inch broad, which 

 we regard as a comparatively gigantic shell, being nearly 

 double the ordinary dimensions. 



The animal varies in colour from pale orange-yellow 

 to tawny, siphons, foot, and mantle being all of the same 

 hue ; the margins of the mantle are firmly and conspi- 

 cuously fringed ; the siphons are united to their extre- 

 mities, where both are surrounded by prominent cirrhi, 

 and the anal one is provided with a tubular valve, which 

 it projects beyond the fringe ; the sides and back of the 

 siphonal tubes are smooth ; the foot is digitiform, and not 

 so large as usual in this genus : it is very extensile, and 

 when the animal is placed on the surface of the sand, 

 it can extend its foot for a great distance, and feel with 

 it in all directions as if with a finger. When in the act of 

 leaping, both foot and siphons are projected very far. 



