This class contains sixty-five cases of presenile dementia–i.e., of insanity ending in dementia, and occurring between the periods of maturity and of senility. Of the 728 cases of insanity under consideration, the present class, therefore, includes 9 per cent., and of the 445 cases of dementia 14.6 per cent. Though attention has already been drawn to the distinction which is necessarily made between “presenility,” an age-period in all individuals, and “prematurely induced senility,” it is perhaps permissible to remark here that cases of the latter type have been included in the class of “senile or ‘ worn-out ‘ dementia,” which was described and illustrated in the last section of this paper.