Variations in the numbers of phalangeal bones in the four-toed salamander, Hynobius tenuis (Amphibia: Hynobiidae), from two localities in Toyama Prefecture, Central Japan.

Variations in the numbers of phalangeal bones in the fingers (I to IV) and toes (I to V) of specimens of Hynobius tenuis collected from two localities (Ashikuraji and Arimine) in Toyama Prefecture, Central Japan, were recorded. In the case of fingers of fore limbs, eight phalangeal formulas were distinguished, and in the case of toes of hind limbs, ten phalangeal formulas were recognized. In the fingers of specimens from Ashikuraji, the predominant formulas were 2-2-3-2 (40%) and 2-2-3-1 (32%). In specimens from Arimine, 2-2-3-2 was predominant (75%). In toes, the predominant formula was 2-2-3-2-0- (96%) for Ashikuraji and 71% for Arimine. The maximal number of phalangeal bones, 2-2-3-3-2, found in the toes of specimens from Arimine seems to represent the fundamental formula in hynobiid salamanders. The predominant formula, 2-2-3-2-0-, observed in the two populations may have been derived from the fundamental formula by degeneration of the phalangeal bones in the fourth and the fifth toes. The reason why phalangeal bones were degenerated in this species is discussed from an ecological viewpoint of body growth.