No. 2.] EYES OF ARTHROPODS. 257 
immature eyes a greater variation is noticeable in the last rows, 
as seen in the following examples of surfaces bearing but ezght 
rows of lenses. 
Gi st 4 De hep idea See aie a 35 
(2) Ay! Me ae he) Se eee Ve 33 
(C) Re PR MR BR ie) ie Tea 41 
(4) COIN. See Ce CR OP NS RR Cee 42 
In these instances the eye has not attained its full growth in 
height, which would preclude variations in the last rows. , 
Conversely, as the first four or five rows of lenses terminate 
on the lower margin of the visual surface, and as additions to 
the number of lenses are made most abundantly from this area, 
the number in these rows is constantly varying. It may be 
here stated, that with the exception of the right and left eyes of 
the same individual, no two eyes in all the specimens enumerated 
have shown the same number of lenses in all corresponding 
rows. 
A definite relation exists between the number of lenses of the 
eyes and the size (t.e. age) of the animal, This fact has been 
established by recording with each enumeration of lenses a 
single measurement which would serve as an index of the stage 
of development attained by the animal. The measurement 
taken is the basal width of the cephalon. Phacops rana is rarely 
found with all the parts in articulation, and still retaining the 
lenses with sufficient distinctness for enumeration. Detached 
cephala are abundant, and it serves every purpose to take the 
indicial dimension from this part of the animal; it is, moreover, 
found that the peculiar form of the cheek renders this dimen- 
sion of the head less liable to variation from flattening in the 
shales than the longitudinal measurement. Comparison of all 
the specimens enumerated gives the following results :— 
The average number of lenses in individuals having a cephalic 
width less than 
TO mm. is 44 
Between 5 and 15 mm. is 56.5 
“ 10 and 20 mm. is 69.5 
ft and\or mim is 92 
20 and 30 mm. is 71 
« 25 and 35 mm. is 66 
