MONTACUTA. 79 



far exceeding any measure of the shell in length, and which 

 is muscular, strong, flat, raised in the centre throughou^t its 

 length, being bevelled to a fine awl-like edge, but its base 

 is very broad, and it tapers to a very rounded termination, 

 not shewing a trace of a lanceolate point. Its locomotive 

 powers would appear to be incompatible with its apparently 

 fixed habitat, if we did not infer that it can detach itself to 

 change place — a power we have observed in other byssal 

 bivalves. When the animal marches the foot is greatly 

 extended, and in this case the rounded termination was 

 instantly fixed to the side of the watch-glass filled with 

 sea-water, in which it was placed. By the muscle of 

 the foot it drew itself forwards, ' itet'umque, iterumque,'' 

 and this manoeuvre was executed with such rapidity that 

 the watch-glass was crossed in a minute. In its passage, 

 the creature, by a twist of its foot, several times turned 

 the shell from one side to the other. Not a trace of sipho- 

 nal processes, or even a simple orifice could be observed 

 in the mantle at either end. When opened for examina- 

 tion the large foot was the most prominent object, and, 

 with a powerful lens, the byssal groove was distinctly 

 visible. Though a pectinated lamina was seen, it was 

 impossible, with the appliances used, to develop the form, 

 or detect the presence of palpi in an animal not one-eighth 

 of an inch in diameter." — Clark, MSS. 



" Mr. Howse's observations on the animal of this species 

 agree with ours on 31. bidentata, as to the shape and 

 position of the foot, and apparent absence of siphons." — 

 Alder. 



Whenever we have met with this curious and scarce 

 shell alive, it has been attached by its byssus to Spatangus 

 purpureus. Though rare, it has a wide distribution. 

 Among corallines in deep water on the Devonshire coast 



