No. 398:] CONES IN THE RETINA OF FISHES. III 
serrata. Among the species that he enumerates as possessing 
twin cones only, there are certainly some in which single cones 
are regularly distributed. He found twin cones only in the 
retina of Scomber, Thynnus, Mugil, Gadus, Platessa, Hippo- 
glossus, Rhombus, Salmo trutta, Coregonus, Clupea harengus, 
and Clupea sprattus. 
He found both twin and single cones in Belone rostrata, 
Tinca, Ammodytes, Carassius, Abramis, Leuciscus. In the 
selachians examined, only rods are to be found. 
Ryder (95) discovered a regularity of arrangement of rods 
and cones in the larva of Salmo salar. The rods referred to 
are the single cones of authors. He believed this definite 
grouping of rods and cones to have some homology with the 
compound eye of the invertebrate. The arrangement he found 
is that of our pattern D. 
Beer (98) in a footnote calls attention to the fact that the. 
cone mosaic is surprisingly plain and regular in Scorpzena and 
Blennius, and can be seen with the ophthalmoscope. He fig- 
ures pattern F. 
Eigenmann (99) calls attention to the regularity of arrange- 
ment in Zygonectes. The pattern noted is Æ, a slight modifi- 
cation of that noted by Ryder in Salmo. He also found that in 
Chologaster the twins predominate over the single cones, and 
that in this genus the arrangement of the twins is a modifica- 
tion of pattern C. 
Finally Eigenmann and Hansell (99) in a short abstract 
state that the twin and single cones are arranged in a regular 
pattern, and that this pattern is either that described by Han- 
nover and by Tot for fishes, or a slight modification of this 
pattern, 
The first notice of a regular alternation of twin and single 
cones isthat by Hannover, and of the arrangement of twin and 
single cones, with a definite arrangement of the axes of the 
twin cones, is that by Ryder. 
The work begun by Mr. G. Hansell under my direction has 
been continued by Mr. Shafer and myself, and a number of 
"Species have been examined to date. Mr. Hansell’ s work is 
incorporated in the present paper, credit being given under the 
