58 W. J. jJAKIN. 



ment. There is a quite irregular series, and a small eye may 

 be followed by another small one or by two large ones, or a 

 group of large eyes may exist together. The eyes on the 

 left mantle-lobe exceed in number those on the right, in 

 particular in species with the most inequivalve shells (as far 

 as the species I have examined are concerned), that is, in 

 V. maxim us and P. jacobteus, and this difference in 

 numbers is greatest in P. jacobaeus. 



The eyes iii this species are far less numerous on the right 

 lobe, and are also very much smaller (PI. 7, fig. 2) (contra 

 Schreiner). 



I believe, however, that the greater number of eyes on 

 the left mantle-lobe is due primarily to the fact that this 

 valve is always uppermost, and not to its shape ; and if a 

 Pecten is turned over on to the left valve, it very soon rights 

 itself by a peculiar turning movement. Patten (22) connected 

 this numerical superiority of eyes on the left valve with its 

 position but was puzzled to see how this could be an advan- 

 tage to the animal, since the eyes on the lower mantle-fold 

 received the light direct from above, and the eyes on the 

 upper one were apparently directed downwards. 



Schreiner (30) also figures them as lying pointed to the 

 ground and at an angle of 45° to the valve. If a Pecten be 

 watched as it opens the valves, it will be seen that the 

 eyes of the left mantle-lobe project just outside the shell, 

 and their field of view is practically as much above the 

 animal as that of the eyes in the right valve. The upper 

 valve is also a little shorter than the lower one, and lies inside 

 it when the shell is closed: the mantle lininor the lower valve 

 is retracted accordingly to a greater extent when the shell 

 is closing. The valves of the almost equivalve species meet, 

 however, ventrally, and the conditions appear either more 

 favourable to the eyes of the right mantle-lobe than in P. 

 jacobaeus, or else, as will be referred to again, this form is 

 an older and more specialised one, and the eyes have begun 

 to degenerate in the lower valve. Some figures are appended 

 which will give an idea of the number of eyes in the three 



