THE EYE OP PECTEN. 57 



different species^ and there is, furtliei", considerable variation 

 among the individual members of any species. 



Carriere (21) stated that those species with large eyes 

 possessed fewer than those with small eyes; that there were 

 always more on the upper mantle-lobe than on the lower; aud 

 that in geueral, large specimens had more eyes than smaller 

 ones of the same species. 



This latter sentence was an important assertion, since it 

 implied growth and development of new eyes during life, and 

 certainly it appeared supported by the fact that large and 

 small eyes exist side by side. 



Patten (22) also pointed out that there were more eyes 

 present on the left valve than on the right, and that they 

 were larger, but he disagreed with Carriere, stating that ii o 

 new eyes develop after a size of 2 centimetres has been 

 attained. Rawitz (25) found similarly more eyes on the left 

 mantle-lobe than on the I'ight, and agreed with Patten on 

 the development. Schreiner (30) agrees also with reference 

 to the number of eyes on the two mantle-lobes, but states 

 that those of the right are not smaller than those of the 

 left (Patten). Had Schreiner examined P. jacobasus, the 

 chief species investigated by Patten, he would not have 

 made this assertion. The eyes are always more numerous 

 on the left mantle-lobe than on the right, as all observers 

 have found. The exact relations, however, vary in different 

 species. The eyes are situated in three groups, on each 

 mantle-fold, one group on the anterior auricular area (two 

 to seven eyes close together), another on the posterior 

 auricular area, close up against the hinge-line, and the third 

 and largest group along the ventral margin of the mantle. 

 Spaces without eyes separate these three regions. In each 

 series the eyes vary considerably in size. Patten (22) asserted, 

 in fact, that a regular arrangement of small and large eyes 

 existed, and Rawitz (25), though denying the existence of 

 Patten's arrangement, stated that a large eye was always 

 followed by a small one. I have examined all the species 

 referred to by Patten and Rawitz and find no such arrange- 



