BIHANG TILL K. SV. VET.-AKAD. HANDL. BAND 24. AFD. IV. N:083. 23 
The jaws. 
OC. poecilopus. || OC. gobio. 
| 
[| 45,2] 57,5] 76,1] 42,5] 57, 
"19 | 43,4] 62,7| 76,1] 42,4 58 | 75,8 
Pirofte ih the upper jaw in 4 of that md 7,8) 7,9). 9,5 7,5) 8,9|10 
| 17 rg ÅTAR RAL DA AN bon 5 21 5l 79 Bal 8 | 33 85 
| serva the lower jaw in 4 of that Fe 11,4 11,2 11,9 11,4 11,6| 12 
FERRARI KOL RUT HECSÅ ae & [10,9] ATT) 213112 
| 
| 
i 
| Length of the body in millimetres . . 
| 
The above table shows that a pretty considerable dif- 
ference exists in the development of the jaws, especially in 
that of the upper one, between young and adult specimens, 
males and females. In youth the mouth is of almost the 
same size in the two sexes; but a difference soon appears, the 
jaws growing considerably larger in the males, but very 
slightly so in the females, for which reason a distinct dif- 
ference between the sexes is noticeable in the older stages. 
These differences due to age and sex have not occasioned 
any great distinction between the two forms in regard to 
the development of the mouth. In middle-sized and adult 
specimens of C. gobio the mouth is usually a little larger 
than in corresponding specimens of C. poecilopus, which is 
in agreement with the other parts of the head, but the dif- 
ference existing is so slight, that if C. poecilopus has not 
attained the same development as C. gobio it has, neverthe- 
less, advanced from its juvenile type. The relative strong 
development of the jaws in C. poecilopus might perhaps be 
explained on the ground that this form, which was developed 
in unfavourable places, where it was difficult to get enough 
food, has needed all possible means to enable it to obtain 
sustenance more easily and amongst these a well developed 
gape. If the mouth in the parent form had been only slightly 
developed, it probably would have proved very unfavourable 
to the existence of the form if the hinderance in the deve- 
lopment which C. poecilopus has generally undergone had 
affected the mouth also. It is possible, however, that already 
, in the parent form the mouth was well developed, which is 
