148 GASTROCILENID.E, 



observable, as the majority of specimens are rubbed and 

 distorted. 



The hinge margin is destitute of any teeth, and is greatly 

 thickened in the more aged shells ; the very immature ones 

 have usually, however, two or three rudimental primary 

 teeth, but these, in most of our English specimens are very 

 minute and caducous. The jDalleal scar is situated very 

 high up, not easily discernible and flexuous. Examples 

 are rarely obtained which exceed an inch in length, and 

 about half an inch in breadth ; the proportions are, how- 

 ever, very variable, as the longer shells are often the least 

 wide ones. 



The animal is oblong, somewhat claviform, or, when the 

 siphons are contracted, oval. The mantle is completely 

 closed, except a small round orifice in the centre of the 

 widest part (that occupying the gape of the shell) through 

 which it can protrude a linear, linguiform, triangular pointed 

 foot, having a byssal groove at its base. This foot is some- 

 times entirely withdrawn, sometimes only protruded as for 

 as the point, and occasionally thrust out to a considerable 

 distance. Mr. Clark has observed a substance lining the 

 mantle, similar to that noticed in Gastrochana, but white. 

 That part of the front of the mantle, where the edges of 

 the valves approach, is brownish and ligamentous. Imme- 

 diately beyond it is the base of the united siphons, which 

 are separated only at their extremities, which are nearly 

 equally, but not quite, on a level. The margins of the 

 branchial and anal orifices are minutely cirrhated ; the 

 cirrhi appear to be simple. There are about thirty around 

 each. The whole animal is more or less tinged with yellow, 

 which colour is palest in the central part of the mantle. 

 The anterior extremity, and the siphons, are of an orange 

 hue, often very bright and intense. 



