410 SAMUEL C. PALMER 
chen fehlen.’”’ The number of elements involved is probably 
greater in eyes of equal size in reptiles than in amphibians, 
because of the larger size of the elements in the latter. 
Detailed enumerations of retinal elements in amphibians have 
not been made, so far as I can learn, but Howard (’08) estimated 
that the rods, cones, and double-cones in the retinas of Necturus 
based on estimates in the fundus, were in the relation 4:1 :1 
respectively; but he made no mention of his method of obtaining 
this result. The outer nuclear layer is said by him to consist 
of a single layer of nuclei in the fundus and a double layer at 
the periphery, each rod and each cone is described as having a 
nucleus and each double-cone, two nuclei. Schultze (’67) believed 
that there was only one nucleus to each double-cone. 
Franz (’05) has found the number of nuclei per square milli- 
meter in the outer nuclear and ganglionic layers in the number 
of selachians. The nuclei, in most cases, were found to be more 
numerous about the center than at the periphery. In Acanthias 
blainvilli the number of nuclei in the outer nuclear layer near 
the center of the retina was 24,000 per square millimeter and in 
the ganglionic layer in the same region 1200; in Galeus galeus 
near the center there were 75,000 nuclei to a square millimeter 
in the outer nuclear layer and 1500 in the ganglionic layer. In 
the deep-sea selachians there was a great increase in the number 
of nuclei in the outer nuclear layer and a decrease in the gan- 
glionic layer. Thus, in the fundus over an area of one square 
millimeter there were in the retina of Chimaera monstrosa 100,000 
nuclei in the outer nuclear layer and only 600 in the same area 
in the ganglionic layer. 
Enumerations of the visual elements in the eyes of inverte- 
brates have been made in some cases. Parker (’90) placed the 
number of facets in an adult lobster’s eye at 138,500 and (’95) 
the number of ommatidia in the eye of an adult crayfish (Astacus) 
at 2500. Hesse (’00) estimated that there were between 2100 . 
and 2400 rods in the eye of Pecten jacobaeus. Among cephalo- 
pods, the retina of Loligo was said to contain 162,000 rods per 
square millimeter and of Scaeurgus only 26,000, with other species 
ranging between the two. 
