[25] BIOLOGY OF THE RHINE SALMON. 451 
difference is often, but not always, caused by the differences in the de- 
gree of development of the ovaria; for the July salmon, which contained 
the smallest quantity of fat (of all those examined by me July 9, 1877), 
had an ovarium which was only developed one-half of that of a very fat 
salmon of August 1. 
This difference gradually vanishes from the beginning of August; the 
female September salmon, but still more the October salmon, are, apart 
from individual differences of bodily build, equally lean as to flesh and 
intestines. 
To follow up these data, as regards the male fish, the measure of the 
length of nose, which, since September 1, 1878, was taken in fish of 
both sexes, is important. While in the female fish the average length 
of nose remains the same, an elongation of the jaw can be noticed in 
most male fish at an early date, and makes it possible to determine the 
sex (with the exception of a small percentage of doubtful individuals) as 
early as April and May. 
Whilst in the male June salmon (length—exclusive of nose—8s00 to 
910 centimeters) the length of the nose, in nine cases out of ten, varied 
between 5.4 and 6.4 per cent. of the weight of the body, and once in an 
exceptional case rose as high as 7.0 per cent., the same or very,nearly 
the same minimum figures occur during the whole of July and August, 
although not very frequently, while the maximum figures become higher 
and more frequent. J'rom September on, and all through October, both 
the minimum and the maximum figures increase, whilst the difference 
of the extremes decreases tosome extent. But even in November there 
is a very considerable difference of length of nose in fully matured fish, 
whose size and shape are otherwise very nearly equal ; and Lam inclined 
to consider these differences as indications of different dates of arrival. 
This will be the proper place to make use of the data relating to the 
numerical proportion between the two sexes, which I obtained from the 
weighing-lists [ had kept without regard to individual differences. For 
1879 I have made my calculations with the aid of the length of nose 
from June 1 whilst for 1878 I did not do this until July 1, at which time 
the sex may easily be determined, even without this aid. The average 
of both years shows as the average proportion of the sexes: 62.6 per 
cent. females, and 37.4 per cent. males. 
