38 ANDERSSON, COMPARISON OF COTTUS POECILOPUS WITH COTTUS GOBIO. 
the lateral line seems to have gradually risen nearly up to 
the back and to have begun to sink again. There is thus 
in the two forms a tendency to lessen the existing dif- 
ference. 
This character seems more than others to suggest that 
a comparatively long time has passed since the two forms 
separated, the lateral line being already at an early period 
very differently situated in these. In this respect C. gobio 
seems to represent the more primitive stage. In C. poeci- 
lopus the lateral line seems to have rapidly approached the 
back after the separation and tends at present to revert to 
its primitive position. 
From the preceding account of the results of my examina- 
tion it appears that the difference between the two forms is 
in general the same as that which exists between specimens 
of different ages and of different sexes, and that the two 
forms developed in the same direction, though sometimes the 
one sometimes the other might be slower in its development 
and thus represents the juvenile stage compared with the 
other. It was usually C. poecilopus which was checked in 
the development, but sometimes C. gobio. We often see-how 
some characters in the two forms have been developed quite 
uniformly and there appears sometimes a tendency to a greater 
difference between them in their following quite opposite di- 
rections of development. 
Recapitulating briefly, we find that C. poecilopus remains 
in the juvenile stage in the following characters: The rays 
in the ventral and pectoral fins are less branched, the pre- 
opercular spine is smaller, the head and its measurements 
are smaller, the eyes are larger, the interorbital breadth is 
less, the distance of the fins from the tip of the snout shorter, 
the appendant membrane behind the second dorsal less deve- 
loped, the tail is lower, and the anal fin lower: all these are 
characters in which OC. poecilopus differs from C. gobio and 
the young fish from the older one. On the other hand, C. 
gobio seems to retain more of the juvenile stage in regard 
to the length of the unpaired fins and the height of the 
dorsal fins as also to the absence of regular spots on the 
