566 William Patten 



pigmented spots, and, in some cases, has a faint wavy outline, each 

 curve corresponding to the outer end of a pigmented celi. The spots are 

 provided with colorless sense cells, for on surface views one sees 

 numerous refractìve and colorless points of varying size ; the light, 

 which is often silvery white, appears to be reflected from the re- 

 fractive granulös within the cells. The refractive points are only (?) to 

 be seen in the colored areas, where, surrounded by and contrasted with 

 the dark pigment cells, they are especially striking. Towards the an- 

 terior edge ofthe mantle, the spots become smaller and, assuming a 

 lineai arrangement at the summit of the Ophthalmie fold, by a series of 

 easy gradations , pass into the faceted eyes , the only kind present in 

 Pectunculus. Being confined at the summit of the Ophthalmie fold to a 

 space about 7 or 8 mm long, they are necessarily less numerous than 

 those oiArca. As in the last uamed genus, the number is greater on the 

 right, than on the left fold of the mantle. For instance, in twelve speci- 

 mens, the eyes varied in number from 1 7 — 25 on the left valve, and from 

 20 — 30 on the right; the average being about 25 and 22 on the right and 

 left mantle respectively. The largest eyes are not situated in the middle 

 of the row, as one might suppose, but at that end farthest away from 

 the hinge. The same tendency to form pairs is seen bere as in Arca. 

 Usually two large eyes are follo wed by two small ones, then two large 

 ones with a small one between ; those of equal size being closest to- 

 gether. I bave observed cases, however, in which as many as six or 

 seven fully formed eyes had united to form one large group, whose 

 origin was indicated by the number of component parts, which had not 

 entirely lost their individuality. 



The eyes of Pectunculus are easily discernible since they are not 

 surrounded by such quantities of pigment as in Arca. There are other, 

 but slight diiferences, which distinguish them from the eyes of the last 

 named genus ; the lens-like rods are larger and more protuberant ; the 

 cover cells of the outer row are thinner and less deeply pigmented ; and 

 the eyes seem to bave arrived at a more stable condition, for there is a 

 diminution in the number as well as variety of intermediate forms ; the 

 few remaining eyes, having reached the height of their development and 

 being sufficient for the requirement of the animai, bave been retained, 

 while the less useful, intermediate forms bave disappeared. 



The eyes may be easily overlooked when the animai is irritated, 

 for the mantle is then contracted in such a manner, that the pigmented 

 outer edge is folded in against the shell, and only the inner side of the 

 thickened mantle, quite devoid of pigment, is seen. But when undisturb- 



