894 THE AMERICAN NATURALIST.  [Vow. XXXV. 
The trunk is essentially the same in the siredon and terres- 
trial states, excepting as to three points, vzz., coloration, the 
dorsal fin, and the ** webbing" of the toes. All of these changes 
have been commented on by previous writers, and may be 
passed without further notice. , 
The post-abdomen is acutely tapering from its origin to 
the tip. Its outline as a whole is very acute, unlike either of 
the Dakota specimens. It bears a very thin and membra- 
neous “fin,” which becomes noticeably wider posteriorly. 
f 
Fic. 4. — End view of gill, showing the gill plates. a, in Colorado forms ; 4, in Amenia specimen. 
This broadening of the fin is greater than anything indicated 
in the figures of axolotls generally, and is quite different 
from the outlines in the two Dakota specimens. 
The internal anatomy of the siredons, excepting of the mouth, 
is strictly the same as that of the mature terrestrial forms. A 
thorough dissection of one and a careful examination of several 
others of the collection demonstrated an anatomical condition 
of the viscera very similar to that indicated by Wiedersheim (79) 
in his Fig. i. The reproductive system in both the males 
and the females is fully matured, and in the females there are 
accumulations of eggs ready for discharge, and the albumen 
