868 
SCANDINAVIAN FISHES. 
260 — 270 mm. the above-mentioned retrogression be- 
gins to assert itself in the changes of growth. The 
four age-classes may therefore be named: nors, stor- 
nors, slom, and storslom. Only in the third of these 
classes have we been able to examine fully comparable 
specimens of all the forms, in order to obtain an ex- 
pression for the sexual differences within the genus. 
One of the most distinct examples of the general 
tendency shown by the development of form within 
the genus — among the many that might be adduced” — 
we find in the averages for the alterations caused by 
growth in the relative length of the suboperculum. 
The said length, expressed in percent of that of the 
head, and reduced to an average for the whole genus 
Osmerus, is 28'5. The several averages, as expressions 
of the form-series and the changes of growth, are the 
following: 
Average in 
Osmerus 
dentex. 
Osmerus 
spirinchus. 
Osmerus 
eperlanus. 
29.8 
28.1 
27.7 
At the age of 
Nors 
— 
— 
30.7 
Stornors ... 
29.8 
(27.9) 
27.6 
Slom cP 
29.21 . 
27.7) 
27.7 1 , 
29.8 
28.5 
>26.6 
„ 9 
80. 4 j 
29.3 ) 
20.6 | 
Storslom _ 
— 
27.9 
25.6 
Here the averages decrease with increasing age, 
and the form-series also shows uniformly sinking aver- 
ages. No constant sexual difference can be discovered, 
for the male averages in 0 . dentex and 0 . spirinchus 
are less, in 0 . eperlanus greater, than the female. 
The case is different, when a constant sexual cha- 
racter exercises its influence on the development. This 
appears, for example, in the relative dimensions of the 
eyes. The vertical diameter of the eyes, calculated on 
an average for the whole genus Osmerus, is 41‘5 % of 
the length of the maxillaries. The several averages 
expressing the difference between the forms and the 
changes of growth, are the following: 
Average in 
Osmerus 
dentex. 
Osmerus 
spirinchus. 
Osmerus 
eperlanus. 
46.1 
37.5 
40.8 
At the age of 
Nors 
48.4 
Stornors 
48.4 
(36.8) 
41.6 
Slom cf 1 
45.4] , 
38.0) 
39.7) 
4».o 
138.0 
•38.2 
„ 9 - 
44.7 1 
37. o ) 
36.8 J 
Storslom 
— 
36.3 
33.9 
a Cf. Smitt, Rilcsmuseets Scilmonider, pp. 168 — 189. 
Here too the changes of growth show sinking aver- 
ages; but in the form-series Osmerus spirinchus occu- 
pies the most advanced rank, as the representative of 
a fairly distinct female character. Should tve desire 
to see how the relation appears when this form breaks 
the series by too high an average, we have a clear 
example in the relative length of the lower jaw. This 
length, expressed in percent of the length of the head 
reduced, and calculated on an average for the whole 
genus Osmerus, is 91 ’9. The averages for the form- 
series and for the several ages are the following: 
Average in 
Osmerus 
dentex. 
Osmerus 
spirinchus . 
Osmerus 
eperlanus. 
89.7 
96.1 
90. o 
At the age of 
Nors 
— 
— 
86.4 
Stornors 
89.3 
(93. i-) 
90.i 
Slom <f 
89.8) 
95.2) 
91.8) 
89.8 
95.7 
92.4 
„ 9 
89.8 1 
96.3 ( 
92.7 1 
Storslom 
— 
98.8 
96.7 
Here we observe a still more prominent sexual 
difference both in Osmerus spirinchus and 0. eperlanus 
— and the former together with the females has the 
highest averages — but Osmerus dentex , the least dif- 
ferentiated form, shows no sign of a sexual character 
in this respect. 
Again, should we wish to see how specific cha- 
racters manifest themselves during the course of de- 
velopment, we can hardly find a better example than 
among the Smelts. The only characters not infringed 
by exceptions, i. e. the only constant specific charac- 
ters — so far as we can gather from our specimens — 
are the following: 
A: 19 — 20 gill-rakers on the lower part of 
the first branchial arch, and at most 29 
on the whole front surface thereof. 
Height (longest ray) of the anal fin at 
most 7 ’6 % of the length of the body Osmerus dentex. 
B : 23 — 25 gill-rakers on the lower part of 
the first branchial arch, and at least 
33 on the whole front surface thereof. 
a: Height of the anal fin at most 7'6 
°/o of the length of the body Osmerus spirinchus. 
h: Height of the anal fin at least 8'4 
% of the length of the body Osmerus eperlanus. 
The manner in which the height of the anal fin 
passes through its changes of growth, and affords one 
