270 POPULAR BRITISH CONCHOLOGY. 



cavation, the animal is content with the two simple, open, 

 and in some cases rudimentary, valves of the shell; and that 

 the tube, whether formed independently or from the ex- 

 panded edges of the valves, is not formed until maturity and 

 completed labour call for repose. Then, like a retired man 

 of business, the mollusc walls in his enclosures, to keep out 

 the agitating currents, and make himself secure and com- 

 fortable for the rest of his days. The British genera in- 

 cluded in Professor Forbes^ family of Gastrochanidce are — 

 I. Gastroch^na. — The body is of an oblong square, 

 rather broadest in the lower end or front, and at the hinder 

 or upper end extended into a long double tube. The canals 

 forming this double tube are one for drawing in, and the 

 other for throwing out, the alimentary matter. The open- 

 ings are both surrounded by hairs, which no doubt serve 

 by their motion to bring nutritious substances within 

 reach. In the front part of the animal there is a small slit 

 in the mantle, through which passes a small, curved, sharp 

 foot, or tongue, very unlike the broad sucker-like foot of 

 the Pholas. The valves of the shell are oblique, white, with 

 a large pear-shaped opening. They are united at the back 

 by a true ligament. The animal, with its valves, is enclosed 

 in a bottle-shaped case, the small end of which generally 



