58 DUNN — PACIFIC COAST AMBYSTOMIDAE [^v^l^VlF* 



ably breeds in them. Therefore the retention of a free columella 

 and the non-development of the operculum is of advantage to 

 the animal. The same thing applies to the entire family Pletho- 

 dontidae, which seems to have had its origin in mountain brooks. 

 Apparently the otic region of the ancestors of the Plethodon- 

 tidae was further advanced at the period at which they entered 

 the brooks than was that of Rhyacotriton (which indeed seems a 

 fairly recent immigrant into that habitat), and hence, the oper- 

 culum has been retained by that family. 



It is interesting to find that, in Rhyacotriton olympicus, the 

 lungs and the ypsiloid apparatus are greatly reduced. This re- 

 duction, which occurs in the whole family Plethodontidae, and in 

 six or eight mountain-brook species of Salamandridae, is here 

 found in an isolated member of the Ambystomidae, which lives in 

 similar situations. 



In connection with this genus I have examined the cranial 

 structure of Ranodon sibiricus, Batrachuperus sinensis, Ony- 

 chodadylus japonicus, Amby stoma ensatum, A. opacum, A. tigri- 

 num, and A. maculatum. Wiedersheim's excellent plates of 

 Ranodon sibiricus and Hynobius naevius, and his descriptions of 

 H. nebulosus and Salamandrella keyserlingii, (Kapfskelat der 

 Urodelen, 1877), as well as Okajima's figures of Onychodactylus 

 japonicus (Zeitschr. f. wiss. Zool., XCI, 3, pp. 351-381, 1908), 

 have been very useful for comparison. In the Asiatic Ambystomi- 

 dae there are two types of vomerine dentition, a continuous or 

 nearly continuous M-shaped series, and a very discontinuous 

 series consisting of two widely separated patches of teeth. The 

 former exhibit a number of transitions between a condition 

 where the prevomers extend back over the parasphenoids and 

 the tooth series extends further back in the middle than on the 

 sides, and one where the prevomers have no such extension and 

 the tooth row is not prolonged in the middle. These extremes 

 are represented by Pachypalaminus and Onychodactylus, re- 

 spectively, and the intermediates by Salamandrella and the 

 species of Hynobius. 



The second type has the prevomer not prolonged over the 

 parasphenoids, and is represented by Ranodon sibiricus and 



