Some of the specific and differential characters of these two species have already been enumerated by Pace (1). Subsequently Norman (2) denied that these two forms were different, so that the literature on them was left in an unsatisfactory state. On investigating these two forms, however, I have obtained sufficient evidence-from the characters of the gonadial tubes, the calcareous collars, and the young of both forms-to verify the observations of Pace and to meet satisfactorily the objections put forward by Norman against them. The differential characters of these two species as given by Pace (loc. cit.) have been verified in an unpublished investigation by Mr. W. De Morgan and by the present writer.