BIVALVES AND CUTTLES. 275 



and by a succession of movements drive themselves a very 

 considerable distance through it, the astonished onlooker is 

 apt to receive something of a shock. It should be noted 

 that the movement is not accomplished by the foot the 

 characteristic organ of locomotion in the Mollusca but by 

 shell and mantle. Nevertheless, just as Anomia has a small 

 attaching byssus, although it attaches itself by another and 

 quite different organ, so the scallop has a foot, although it is 

 not used in locomotion. When your little scallops lie 

 motionless at the bottom of the dish, you may see the 

 slender, finger-like foot protruded at one side. It is capable 

 of spinning a slender byssus, by means of which the animal, 

 especially when young, temporarily anchors itself. 



When you have studied the living animal, and watched 

 its curious flight, you should collect a goodly number of 

 shells from the shore and proceed to study them in detail. 

 It is well to have a considerable number of specimens, for 

 there is a large amount of variation, especially in colour 

 indeed, there are said to be no less than six colour varieties 

 in the Firth of Forth alone. 



If your specimens consist, as often happens, of detached 

 valves, you should first pick out and distinguish the upper 

 and lower valves. Both valves are convex, but one is more 

 convex than the other, and in natural conditions it is the 

 less convex which is the upper. The difference between 

 the two valves is, however, not marked, and the shell is 

 therefore described as sub-equivalve that is, with nearly 

 equal valves. It is also almost circular (sub-orbicular) and 

 almost equilateral. Like that of all other Pectens, or 

 scallops, it is furnished with two ears, here almost equal 

 in size, and has a straight hinge line with a marginal 

 ligament, and a central cartilage placed in a pit beneath 

 the beak of each valve. The special characteristics of the 

 species are found in the number of the ribs (about twenty) 

 and the peculiar structure of the surface of the shell. It 

 is covered with close rows of minute scales, which require 

 some little attention before they can be seen, but once the 

 shell has been closely studied it is almost impossible after- 

 wards to mistake the species for any other. In the 

 commonest colour variety the shell is red-brown marked 

 and spotted with white. 



