548 William Patten 



perfectly round , black holes , separateti from each other by chocolate 

 brown pigment. The statement of Will is perfectly justifiable, that 

 »one might name them aggregate eyes, for they bave ali 

 the characteristics ofthose compound eyes, which we are 

 accustomed to designate by that name.« 



In one example oi Arca Noae^ 8,5 cm long, I bave counted 133 

 faceted eyes in the leftmantle edge, and 102 in the other. In the upper 

 edge, I bave counted 30 in the first centimetre, and 37, 18 and 10 

 successively in the three following. Then comes a wide space without 

 faceted eyes, succeeded by three centimetres. each containing 14. 20 

 and 4 eyes respectively. 



I bave said that these eyes were arranged at perfectly regulär in- 

 tervals, tbis is, however, not strictly true, altbough, when observed 

 with a hand lens, such is the general effect produced. One often sees 

 two eyes so dose together that they form one large, ovai and double 

 eye spot (fig. 41). This occurs so frequently that, in one individuai. I 

 have counted four or fìve such double eyes in various stages of union ; 

 from one in which they form two eyes, so near together as to touch each 

 other, to those in which the double origin is only indicated by a slight 

 departure from the usuai circular outline, and by a shallow, hardly per- 

 ceptible furrow in the middle. Now, since the single eyes vary consi- 

 derably in size, it is difficult to determine in ali cases whether some of 

 the larger are formed by the fusion of two smaller, — the traces of union 

 having entirely disappeared — or whether some of the latter were 

 originally formed by the concrescence of two stili smaller eyes . On the 

 branchial side of the Ophthalmie fold, that is on the side towards the 

 light, the hypodermis with its cuticula is especially thickened and con- 

 tains numerous isolated, or scattered ommatidia in a high stage of 

 development (figs. 46 and 47). 



In Arca barbata^ I bave found, in a specimen measuring 5 dm in 

 length, 91 faceted eyes on one side of the mantle, and 83 on the other, 

 making in ali 174, about the average number. 



In Arca tetragona^ 10 to 15 mm long, there are from 25 to 30 

 faceted eyes in each mantle edge. 



Each eye is situated in a triangulär, pigmented area; in the thickest 

 part of the mantle, the areas fuse with each other to form a continuous, 

 pigmented layer. The isolated areas differ in the intensi ty of their color- 

 ing; they are usually Vandyke brown, and contain a number of darker, 

 irregulär, pigment spots, largest and most numerous in the immediate 

 vicinity of the eye. In some cases these spots are absent. 



