1 8 J. H. Powers 



olds by the same means ; older animals are not only different in 

 these respects, but become much scratched and worn by burrow- 

 ing. The skin about head and jaws becomes loose and wrinkled; 

 both the character of the pigment and the color patterns become 

 altered ; while the most definite, if the most trivial, means of 

 estimating age is by the number of notches in the dorsum of the 

 tail, due to the bites of rodents. Young animals never show many 

 bites, while those that are much worn, etc., show the dorsum of 

 the tail completely serrated. Such animals can hardly be less 

 than ten to fiftieen years old and are very probably more. Judg- 

 ing of age by these means, the forms of very old specimens do 

 not prove to be peculiar, and there is no evidence of continual 

 elongation of the tail save in very rare instances. 



Finally, it is of interest to note that this same sudden elonga- 

 tion of the tail accompanies the assumption of sexual maturity 

 by the male amblystoma in the branchiate (so called larval) con- 

 dition. I have not been able to observe directly the development, 

 because I have not been able, despite repeated trials, to develop 

 sexually mature male aquatics in aquaria. But larvae placed by 

 me in a reservoir have attained a superb sexual development, even 

 so soon as at the close of their first summer ; and by comparing 

 the males among these specimens with the females, with ordi- 

 nary larvae, and with immature males from the same source, it 

 was quite evident that the peculiar elongation of the tail had 

 taken place just as it would have done had metamorphosis pre- 

 ceded the assumption of the breeding condition. The largest of 

 these males measured 29 cm. in length, and over half of this 

 length was comprised by the tail. These specimens had grown 

 slowly for the first half of the season, consequently assumed the 

 slender form, and the sexual maturity of the males had brought 

 the tails up to almost the extreme length. 



A certain special interest attaches to this assumption by the so- 

 called larval, or axolotl form of this secondary sexual character. 

 Whether or not it actually is assumed by the Mexican axolotl I 

 have been unable to ascertain. But the development of this char- 

 acter by our sexually mature aquatic amblystoma shows (to- 

 gether with many other features, such as size, coloration, etc.) 



214 



