166 PROCEEDINGS OF THE ACADEMY OF [1895. 



eye, where the single visual organs are known as ornmatidia. This 

 distinction is a necessary one in view of the fact that it is obviously 

 impossible to assert, in the face of the morphological evidence, that 

 there is an exact homology between what are known as ornmatidia and 

 the cell-groups in Salmo that I have called retinidia. I have found 

 it difficult to determine the exact number of rod-cells in the center 

 of the groups. As nearly as can be made out with an immer- 

 sion y, there are five. At first when this regularity of structure of 

 the salmon's retina was noticed with low powers it was supposed that 

 there was only one rod- cell in the centre of each retinidial group. 

 The overlapping or blending of these groups with one another in 

 Salmo is shown in both Figs. 1 and 2, and this is the one feature that 

 makes it impossible to homologize them exactly with the ornmatidia 

 of the compound eyes of Arthropods. 



