606 William Patteu 



eompares to those found by Fraisse in Patella, aucl believes, witb bim, 

 tbat the lattei* scerete the cutieiila; wbile tbe former are tbose sensitive 

 to lig-ht. 



Mactra. 



The extremity of tbe sipbo of Mactra stultorum is provided witb a 

 number of ovai pigmented areas, varying in color from madder brown 

 to dark purple, or even jet black; some, usually the darkest ones, are 

 placed at the base of the tentaele, others, sìmilar in shape, but lighter 

 colored, are to be found on the free edge of the sipho. In the latter 

 case, the pigment cells are arranged in small groups, in the centre of 

 v^hich is a clear, refractive spot. At the base of the tentacles, the cells 

 are too deeply colored to admit of such a clear view. In the latter in- 

 stanee, it appears as though the pig-ment was deposited beneath the 

 surface , but , unfortunately , the superficial examination that I gave 

 them will not allow me to make any positive statement : I, however. 

 believe that the Clusters of pigmented cells, in the centre of wbich were 

 the clear refractive points, are the same structures we bave seen in 

 Arca, that is ommatidia, composed of pigmented cover cells sur- 

 rounding a centrai colorless one. 



In Mactra hehetica, the pigmented areas are less numerous but 

 apparently more highly speci alized. 



Sharp has examined Mactra solidissima and found the same scusi- 

 ti veness to light and shadow, as in other genera witb which he has ex- 

 perimented. I bave tested Mactra stultorum and Mactra helvetica, in 

 the same manner, and found that they also bave this sensitiveness 

 to a marked degree. His statement that the pigment cells tend to form 

 shallow grooves at the base of the tentacles, I am neither able to 

 confirm nor refute,] having never studied them by means of sections, as 

 he has done. 



Pinua. 



The bodies on the mantle oiPinna, first described by Will as eyes, 

 bave in more recent times been supposed to possess some other function 

 than that of vision. That they bave the general shape and appearance 

 described by Will, can easily be seen on superficial examination. The 

 slit-like fold, which he considers to form a kind of iris, is caused by 



