SEA-FISIERIES LABORATORY. 345 
being nearest the hinge line and the base furthest from it. 
When the valves are closed the ligament is compressed 
and the free surface becomes very convex, so that the 
shell is only kept closed by the adductor muscle over- 
coming the resistance of the ligament. It will be noticed 
in both species that when the valves are closed there are 
two places, one on each side extending from the hinge line 
to the greatest antero-posterior diameter, where the shell 
edges do not meet. It is through these two prominent 
gaps that the water is forcibly ejected in swimming. 
Owing also to this feature, sea water cannot be retained 
in the pallial cavity when the animals are removed from 
their natural habitat, and hence Pecten lives but a short 
time compared with the Mussel and the Oyster, when 
exposed to the air. 
In P. maximus the convex valve overlaps the flat valve 
by from one-eighth inch to one-quarter inch when they are 
closed. Jeffreys describes the hinge plate in P. maximus 
as toothless, but mentions certain ridges present on it. 
There are several tooth-lke ridges both on the anterior 
-and posterior sides of the Lgamental pit, and these inter- 
lock when the valves close, fitting into grooves between 
similar ridges on the other hinge plate. They are not 
developed in P. opercularis. here are, further, two 
prominences on the right valve just at the point where the 
auricular areas meet the main portion of the valve 
(Pl. I., fig. HE, Sh.p.). These two tuberosities rest in two 
depressions on the left valve when the shell is closed. 
In both P. maaimus and P. opercularcs the auricular 
areas are almost equal in size, and in the former almost 
similar in shape, with the anterior and posterior margins 
inclined slightly, making an obtuse angle with the hinge 
line. In P. opercularis the posterior edges incline, 
making an obtuse angle as in P. maaimus, but the anterior 
