558 THE AMERICAN NATURALIST. [VoL. XXXIV. 
between 38 per cent and 30 per cent, in all cases about 5 per 
cent greater than for adults of A. tigrinum of the same total 
length. The distance between the centers of the eyes is less 
than in the adult Amblystoma, where the eyes, instead of 
being located considerably inside of the margin of the face, are 
lateral. The area of the face, in front of the line connecting 
the centers of the eyes, is also less than in the adult. In the 
latter the distance from the snout to this line is generally about 
one-third of the length of the head, while in the Dakota form 
it is one-fifth. The diameter of the mouth opening, too, is less 
than in the adult of A. żigrinum, in that the angle of the 
mouth coincides with the angle of the jaw; in other words, 
there are no developments of free skin, forming lips or cheeks, 
and the angle of the mouth is located 
directly under the eyes; but here, 
while the angle of the jaws is beneath 
the eyes, there are folds of skin form- 
ing lips and a sort of cheek cavity, so 
that the angle of the mouth lies ante- 
rior to the level of the eyes. 
On the under side of the front part 
of the head there is a transverse fold 
of skin, “gular fold," which is at- 
tached in front and free behind, where 
it overlaps the gill slits and bases of 
the gills. This fold is entire in the 
fig Tiari pg eng eg middle line, as seen in Fig. 3, in con- 
with the gular fold (g,/.) turned for- trast with its decidedly emarginate 
irs): Vi gi EE Sed on outline, as seen in Baird's figures 
oe (59, Figs. 1 d, 2 c) and in the Montana 
specimens. The gular fold covers and hides four gill slits. 
The first one is directly behind it and in front of the first of 
the four gill arches. It differs from the other three slits by 
the absence of denticulations of the skin, reinforced by carti- 
lage, and reminding one of the gill rakers of the sturgeon, 
which guard the passage through the other three slits. The 
arches are supported by a rod of cartilage which arches around 
1 But not in Baird, '52. 
