12 ANDERSSON, COMPARISON OF COTTUS POECILOPUS WITH COTTUS GOBIO. 
As shown in the table above, the tail of OC. gobio is in 
most cases deeper than that of C. poecilopus, though there 
appear great varieties in both species, and there is no definite 
distinetion to be detected between the individual specimens. 
In C. gobio the relative measurements vary between 7,3 2 
and 4,4 2; in C. poecilopus between 5,8 2 and 4,1 «4. These 
measurements differ considerably from those which HECKEL 
gives. He says, however, that the tail in C. gobio is from 
1/12 to 1/13, being 8,3 2 to 7,8 2 of the length of the body; 
and in OC. poecilopus !/14 or 7,1 2. This difference may well 
be due to the difficulty of taking exact measurements of the 
tail. It is possible, however, that in specimens from the more 
southern latitudes the tail may be deeper. (In the Danish 
specimens the tails were very deep, in the gobio-specimen 
6,8 2 and in the poecilopus-specimen 5,8 2 of the length of 
the body.) 
Besides the above-mentioned important characters, HECKEL 
gives in his more detailed description of the species the fol- 
lowing distinctions between C. gobio and C. poecilopus. The 
distance between the eyes is greater in C. gobio (by about the 
diameter of the eye); the spine of the preopercle is turned 
somewhat more upwards in OC. gobio; the pectorals and the 
innermost ray in the ventrals are longer in OC. gobio. 
The different breadth of the interorbital space. 
Professor LILLJEBORG also points out this character in 
his specific diagnoses, though he expresses it differently, say- 
ing that the breadth of the frontal bone at its narrowest 
part is more than half the length of the vertical diameter 
of the eye in C. gobio, but less in C. poecilopus. As LILLJE- 
BORG's method of measuring the distance between the eyes is 
easier and more trustworthy than that of HEcKEL, I have 
followed the former, and found by it that only two of my 
51 specimens of OC. poecilopus exceeded the stated maximum 
measurement, but that in more than half (37 out of 68) of the 
specimens of C. gobio the distance between the eyes was less 
than half the vertical diameter of the eye, being the same 
as in OC. poecilopus. The interorbital space increases with 
age, but I found some even amongst the largest specimens of 
C. gobio, in which the interorbital space was narrow. Although 
