Abstract
It is particularly difficult to distinguish between M. ikonnikovi and M. mystacinus based on external taxonomic characters because of their extreme similarities in morphology. Although they may be distinguished easily based on their skulls, it is not permitted to take specimens for identification because M. ikonnikovi is classified as threatened and endangered and M. mystacinus is classified as threatened and vulnerable, by Japan's Ministry of the Environment. The authors describe an entirely new method of identifying live individuals based on an external character, in particular the pattern of the blood vessels visible in the tail membrane: in M. ikonnikovi the main blood vessel forms a 'Dog-leg type', whereas in M. mystacinus it forms a 'Straight type'.
