314 
A. E. Verrill — North American Cephalopoda. 
The pupils of the eyes are deep bluish black ; on the upper side 
they are encroached upon by a sinuous, downward extension of the 
iris, which is silvery or pearly white, with brilliant, green, opalescent 
reflections at the upper margin. 
Sexual differences. 
The sexes differ to a considerable extent, in proportions. If we 
compare specimens of equal length, the female will have the body 
relatively stouter and less tapered posteriorly than the male ; the 
head is decidedly larger;* the arms are longer; the suckers are 
usually distinctly larger, especially those of the tentacular arms. 
But if we compare specimens having the head and arms of equal 
size, the male will be found to have a decidedly longer, more slender 
and more tapered body, and a somewhat longer and narrower fin. 
(See table B, for comparative proportions.) 
In the adult male the circumference of the head to the mantle- 
length usually varies from 1 : 2*55 to 3*45, averaging about 1 : 3T0 ; 
in the female from 1 : 1*75 to 1 : 2*45, averaging about 1 : 2-25. 
The ratio of the breadth of the fin to the mantle-length, in the 
male, varies from 1 : 2*12 to 1 : 2 - 45, averaging about 1 : 2*25 ; in the 
female, from 1 : 1*70 to 1 : 2*12, averaging about 1 : 1*90. 
The ratio of the diameter of the largest tentacular suckers to the 
mantle-length varies, in the male, from 1:50 to 1 : 90, averaging 
about 1 : 65 ; in the female it varies from 1 : 36 to 1 : 54, averaging 
about 1 : 45. 
The proportion of the length of the dorsal arms to the mantle- 
length, in the male, averages about 1 : 3*50 ; in the female about 
1 : 2*75. 
The most marked effect of strong alcohol is to reduce the diameter 
of the body and the breadth of the caudal fin to a proportionally 
far greater extent than it does the length of the mantle and fin. 
Therefore, specimens that have been preserved in too strong alcohol 
often look like a different species, and the females often resemble the 
males, on account of their apparently longer and narrower fins and 
unnaturally slender bodies. 
The pen of the female is relatively broader and shorter than that 
of the male (see table A). 
* Some of the nominal European species of Loligo, that have been based on the 
smaller size of the head, arms, and suckers are probably only the males of the common 
species. The sexual variations in this genus have apparently been very imperfectly 
understood by European writers generally. 
