No. 419.] THE ANATOMY OF AXOLOTLS. 891 
individual variation, a more accurate statement of the amount 
of difference could be made. But if we compare a Colorado 
specimen with a St. Paul specimen of the same size, we find 
in No. 1 of the Colorado forms a ratio of 32%, while Ambly- 
stoma, No. 8, of the same size has a ratio of 28%, and the 
larger St. Paul specimens have a still smaller ratio than even 
larger specimens of the Colorado series. Thus No. 7 from 
St. Paul has a ratio of 24%, while the largest of the Colorado 
series, a much larger animal, has a ratio of 32%. One should 
not, however, place much reliance on the comparison of indi- 
viduals, but I believe that on large series of individuals of the 
same size it would be found that the head ratio in the siredons 
. is much larger. 
Not only is the head as a whole larger in the aquatic forms 
than in the terrestrial ones, but the proportion of its parts is 
also different. A reduction of the head in the throat region 
would be expected in connection with the degeneration of the 
branchial apparatus, and does occur. This is seen by reference 
to the following statement showing the lengths of these regions 
of the head in a number of different Colorado and other sire- 
dons and in six St. Paul amblystomas, 775. : 
COLORADO SERIES | St, Papi Sen 
No, Shes Snout to T y No Sizes E uu 
12 f 27% 73% - u 41% 59% 
II É 26% 74% 5 1 41% 59% 
5| 4 26% 74% 8 36 40% n 
ES A 26% | 74% | 1 | H "T ln : 
Ado 35% | 65% || 12 | d 35% 7 
; Hn 33% 67% 4 ho dove pov 
D 27% 73% . 60.3% 
. Hou oz €— 
E xx a 
0 o 
9 35 23% 77% 
8 
13 m 28%, 72% x | H 22% 7 y 
Average . ee EAE ee 
R eee: r KENMARE SPECIMEN 
Meram ae eer ee ug 
- m SPECIMEN r 4 | 29% | 73% 
Ex i | 284, | 72% i RM 
