No.398. CONES JN THE RETINA OF FISHES. II3 
to each other. This pattern was found in Sedastodes elongatus 
(Fig. 2). A similar but less regular arrangement was found by 
Eigenmann (99) in Chologaster cornutus (Figs. 3, 4?). In the 
latter case there are also some single cones irregularly disposed 
among the twins. 
Pattern D is formed from 
4 by the addition of a sin- 
gle cone in the center of 
the square. This pattern has 
been known longer than any 
other, and is evidently the 
one described by Müller for 
Perca, in which he says “ each 
single cone is separated from 
its neighbor by twins." It is D 
found in the American and 
the European species of Perca (Fig. 5), in which the single 
cones are of nearly the same size as the components of the 
twins, and in Micropterus (Fig. 9). It was also found in 
Etheostoma caeruleum (Fig. 6), Apomotis cyanellus (Fig. 7), and 
Pimephales notatus (Fig. 8). The three last eyes were pre- 
pared and the figures drawn 
by Mr. Hansell. 
The closeness of the ele- 
ments in the pattern and the 
size of the elements are seen 
to differ in the various species 
examined. 
Pattern Æ is but a slight 
modification of pattern D, in 
which some of the twins have 
pg migrated along the faces of 
the squares so that while their 
axes, if extended, still form squares, the lines separating the 
twins, if extended, form parallel lines instead of a continuous 
line. This condition was found by Eigenmann (99) in Zygo- 
nectes (Figs. 10, 11). 
Pattern F is formed by adding a single cone at each angle 
