It refers to the fact that these rodents — literally the "middle mouse" — were considered an intermediate form between the genus Loncheres and the genus Echinomys, both of which are now known to be synonyms of Echimys.
It pertains to the fact that these rodents — literally the "middle mouse" — were regarded as an intermediate form between the genus Loncheres and the genus Echinomys, both of which are now recognized as synonyms of Echimys.