890 THE AMERICAN NATURALIST. [VoL. XXXV. 
opening, we obtain the ratio of head length to head and body 
length, and can then make direct comparisons, irrespective of 
difference of length. By measuring to the cloaca we avoid 
the influence of the variation of the post-abdomen. These 
measurements and the ratio are shown in this table. 
Ratio or HEAD LENGTH TO LENGTH OF HEAD AND Bopy. 
COLORADO SPECIMENS dx hui AMENIA SPECIMENS 
AMBLYSTOMA M 
No. | Sizes Ratiol No. | Sizes Ratio| No. | Sizes Ratio|| No. | Sizes| Ratio| No. | Sizes | Ratio 
12 | 48 |37%|| 1 | v |32%|| 10 |f 12776|| 12 | 33 |32% X | fx | 31% 
$ |36 6 2 Serer a) aes 
3; ü 3 % Yos 31% 4 "n 8% KENMARE SPECIMENS 
5 | 1$ 335 2 | wus |29% 8 | fus | 28% || —— m 
3| 8 |342|| 7 | 3s |30% 5 | e's | 26%|| x | ates | 29% 
8 | $8 |37%|| 9 | Ahr | 31% 10 | 49s |27% 
13 | fs |32% | 7 |2 | 24% 
Average 36% || Average 31% | Average 27.5% 
These figures show great variation in the head length in 
general among these forms, so that inferences from single 
cases will need to be carefully guarded. Thus in the first 
group we find ratios of 33% to 37%, and in the St. Paul forms 
we find a range of from 24% to 32%. The size variation thus 
shown to exist would be an interesting topic for study, and has 
not as yet been studied. Referring now to the ratios, we see 
clearly that on the whole the Colorado forms have a larger 
head than the St. Paul forms. In the latter the ratio is 27.5%, 
while in the former it is 31% for the larger sizes and 36% for 
the smaller ones. These figures are in reality an underesti- 
mate, for they are reduced by the fact that in the comparison 
no allowance has been made for the tendency of smaller speci- 
mens to have larger heads. This tendency is very well shown 
by comparing specimens of different sizes in the foregoing 
statement. Thus in the St. Paul specimens it is very clearly 
seen, and in the Colorado forms the smaller ones have a ratio 
of 36%, while the larger ones have a ratio of only 31%. If the 
series of specimens were large enough to enable us to eliminate 
