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.
This pertains to the classification where these rodents, referred to as the "middle mouse," were considered a transitional species between the genera Loncheres and Echinomys, which are now recognized as synonyms of Echimys.