892 THE AMERICAN NATURALIST. [Vor. XXXV. 
These figures show that the average ratio of the length of 
the hinder part of the head to the front part is 72% in the 
siredons and only 60 in the terrestrial amblystomas. 
The form of the outline of the head anteriorly is another 
point in which the aquatic and terrestrial forms differ con- 
siderably. The difference is shown in Fig. 2. In the former 
the outline of the head in front is very blunt and broadly 
convex; in the latter the outline bends suddenly after the 
Fic. 2. — Dorsal view of head of Colorado siredons, No. 2 and No. 1o. 
nostril is reached, producing a sharply convex curve. This 
is due to the presence in the aquatic form of a cheek fold of 
the skin bordering the jaws, not found in the terrestrial state. 
In the terrestrial forms the skin is drawn and tight-fitting over 
the whole of the head. This is obviously correlated with the 
need of providing against the desiccating influences of the 
atmosphere. In the aquatic forms, on the other hand, the skin 
is very loose and ample. This is noticeable generally in the 
head, not only in the general skin, but in the presence of gills, 
opercula, and such special modifications of the outer surface. 
