22 ANDERSSON, COMPARISON OF COTTUS POECILOPUS WITH COTTUS GOBIO. 
of the forms; but, as well as other examined measurements 
of the head mentioned above, this also shows that C. poeci- 
lopus remained in a more original state of development. We 
find, namely, that the eyes diminish in size with age and 
that those of C. gobio are smaller than those of C. poecilopus, 
which very clearly” shows that the same correspondence exists 
between OC. poecilopus and OC. gobio as between young speci- 
mens and older ones. Where the changes due to age show 
an increasing ratio, the percentage in C. poecilopus is less 
than in OC. gobio and vice versa. 
An exception to this rule seems at the first glance to 
appear in the breadth of the cheek, which usually diminishes 
with age, yet is, however, broader in C. gobio than in C. 
poecilopus. Nevertheless, if we look into the case more deeply, 
we find that there exists another cirecumstance which changes 
the normal course of development. 'The breadth of the cheek 
is the distance between the angle of the mouth and the hind 
margin of the preopercle, for which reason it is elear that 
the size of the mouth or the length of the jaws has a great 
influence upon this measurement. Respecting the development 
of the jaws (spoken of on p. 23) we find the following: The 
jaws are about equally developed in the two forms and are 
considerably larger in the males than in the females. The 
anterior part of the head being smaller in C. poecilopus than 
in C. gobio and the mouth equally developed, it is evident 
that the portion of the anterior part of the head which is 
situated behind the mouth must be smaller in OC. poecilopus. 
The reason why in the males the size of the cheek diminishes 
with age may be that the mouth in these increases in growth 
to a greater degree than the anterior part of the head does, 
causing a decrease with age in its hinder portion. The same 
state of things appears also in the females of C. poecilopus, 
in which the decrease of the cheek is very inconsiderable, 
because -the increase of the jaws is very much less in these 
than in the males. On the other hand, in the gobio-females, 
whose jaws increase still less, we even find « an increase of 
the breadth of the cheek. 
