No. 403.] A REMARKABLE AXOLOTL. 559 
from the hyoid bone to the roof of the skull. The skin below 
the arches forms a thin flap, the operculum, which is continu- 
ous at its outer edge with the gill, a fleshy outgrowth running 
outward and backward. The gills are compressed at the base 
so as to be flattened vertically; but this decreases as you pass 
out on the gill, till at the tip it is flattened from above and 
considerably widened to a spatulate form. They are of differ- 
ent lengths, increasing as they go back, and the longest is 
considerably longer than the head. In all other forms they 
are no longer than the head, and in most they are decidedly 
shorter. And in all other forms they are lanceolate in outline 
and taper acutely at the tip. 
The gills bear a tangled mass on the under side, or at the 
margins in the flattened distal part, composed of a great 
number of jet-black, long, and slender filaments, some of which 
attain a length of 11 mm. The filaments are actually much 
more numerous than is shown in the figures, forming an indis- 
criminate mass of hairy appearance all down the edge of each 
gil. These filaments, again, are 
unlike those of other forms. Thus 
in S. gracilis of Baird (59) the fila- 
ments are broad, flat, short, and 
straight; here they are slightly 
flattened, but are extremely long 
and slender. In the Montana 
Specimens these structures are 
more suitably indicated by the 
term * plates" and are I mm. 
wide by only 3 mm. long. . m 
The mouth cavity is illustrated  % teeth of lower jaws w2, upper Ip. 
in Fig. 4. There are thin, slightly 
fleshy upper and lower lips. Directly behind the upper 
lip is a bony arch, bearing a limited number of small teeth, 
mx., which are set close together and crowded so as to form, 
somewhat indefinitely, two rows. This arch is relatively much 
shorter than it is in the adult A. tigrinum, where the row of 
maxillary teeth reaches back nearly to the angle of the jaw, 
and very considerably posterior to the level of the posterior 
