This paper is based on the comparative study of 15,516 specimens of Taphius glabratus from 36 localities and 12,776 of Taphius tenagophilus from 31 localities. A description of the renal region of both species is given, and it is concluded that they are distinguishable by the presence (in T. glabratus) or absence (in T. tenagophilus) of a ridge along the middle line of the membrane which covers ventrally the renal tube.
The ridge makes its appearance in specimens of about 10 mm in shell diameter and then gradually develops as the animal grows. It rises from a pigmented stripe which represents its forerunner in the small specimens under 10 mm in diameter.
The renal ridge is defined, both anatomically and histologically, and is shown to be a reliable diagnostic character of T. glabratus subject to age variation.
The unridged renal membrane of T. tenagophilus is also defined, with especial reference to its variation and to some unusual characteristics that may lead the taxonomist to confuse T. tenagophilus and T. glabratus.