No. 403.] A REMARKABLE AXOLOTL. 557 
as I can tell from figures, from that of the other larval forms. 
It is not smooth, but is somewhat coarse and warty-roughened, 
generally on the back and sides; but this condition is replaced 
on either side of the dorsal fin by a narrow area in which the 
skin is entirely smooth. This difference in texture is notice- 
able. It is not shown in Baird’s figures, but it is possibly 
indicated in Marsh’s, so that it may exist in some stages, at 
any rate, of their forms. 
The head in the Dakota form and in all the siredons is quite 
different from that of the adult A. ¢igrinum. — Dorsally it is flat- 
tened as in A. Zigzinus, but ventrally it is swollen and rounded 
SO as to present in a side view a very different appearance ; 
this difference is directly correlated with the presence of the 
branchial apparatus. The whole head, however, is relatively 
larger here (and in the Montana form) than in the adult A. 7zgr:- 
num. The measurement of the length of head is made from 
the line connecting the posterior boundary of the gill; the 
location of this is still visible as a rudiment in the adult, and 
thus furnishes a very constant point from which to measure. 
For comparison with this I have taken the distance to the 
hinder boundary of the cloaca, which gives the length of the 
trunk, to compare with that of the head, as being more accu- 
rate than to take the total length. There is some variation 
in the sizes of all these regions, and by taking the head and 
comparing it with the trunk the influence of variation is less 
than it would be if the two regions of trunk and post-abdomen 
were both taken. 
The length of the head in the Dakota specimen is 31 per 
cent of the length of head and trunk together. This is rela- 
tively larger than the head of A. tigrinum in the larger sizes 
of the species. Thus it is found in a series of twelve speci- 
mens that the length of head ranges from 31 per cent of head 
and trunk, in the shortest cases, to 25 per cent in the longest 
ones. If this ratio were followed out, the length of head for a 
specimen 312 mm. in length would be only 20 per cent of head 
and trunk, in contrast with 31 per cent as here. This point of 
Size of head shows even more distinctly in Dr. Lee's Montana 
forms than in this one from Dakota, for there the range is 
