SEA-FISHERIFS LABORATORY. 337 
its valves, causes the shell to open and close in a very 
rapid manner, and it might be thought that at each 
sudden clapping of the two valves, the water between 
them would be forced out ventrally and that the animal 
in consequence would move with the hinge line foremost. 
The free or ventral border of the valves is, however, 
directed forwards in swimming, and the animal seems to 
take a series of bites at the water. 
As will be subsequently described, the valves in both 
species are not mirror images of each other. Pecten 
maximus has the right valve very much more convex, 
while the left is quite flat. In P. opereularis the two 
valves are much more alike but the right is slightly less 
convex than the left. Pecten maximus lies on the convex 
valve or right side, and the flat side is, therefore, superior, 
and is generally covered with barnacles, serpula, 
zoophytes, &e. LP. opercularis also shows by the attached 
animals being found always on the same side, that it lies 
on the right valve. If a specimen is turned over on to 
the other side, it will make efforts to turn back, and 
usually regains its normal attitude in a few minutes. 
The two diagrams in text (fig. 1) show that while the two 
species both he on the right valve, in one case the more 
convex side is downwards, and in the other, upwards. 
If the undisturbed animal is watched as it opens the 
valves (which it does very slowly), the tentacles (Plate 
II., fig. 1, Zn.) will first be seen gradually protruded, then 
the eyes will become obvious, and lastly, when the valves 
are some distance apart, the two free edges of the mantle 
(which previously lay against the mantle lobes proper) 
move outwards until they stand almost at right-angles to 
the plane of the valves, so as to form one curtain or 
“velum” (fig. 1, V.) hanging from the upper valve and 
one projecting up to meet this from the lower valve. 
