18 



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-like ridges both on the anterior 

 and posterior sides of the ligameutal 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. o'perc^ilaris. There are, further, tM^o 

 prominences on the right valve just at the point where the 

 auricular areas meet the main portion of the valve 

 (PI. I., fig. E, Sit. p.). These two tuberosities rest in two 

 depressions on the left valve when the shell is closed. 



In both P. tn(tx/}niis and P. open-idaris 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. ojyerculai'is the posterior edges incline, 

 making an obtuse angle as in P. ma.rimvs, but the anterior 



