314 A. E. Verrill — North Atnerican Cephalopods. 



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 arras. 

 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 : 3-10 ; 

 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 : rVO 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 : 60 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 LoUgo, 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. 



