In the present paper I propose to give the results of a series of investigations carried on in the Zoological Laboratory of the University of Aberdeen. Through the kindness of Professor Nicholson, I have had access to his admirable collection of specimens of Solenopora, Dyb., and I have also had the privilege of using his extensive series of microscopic preparations of the same. With such material, and with additional preparations, I have been enabled to examine, not only species already described, but also a number of new species, of which I shall in this paper give brief descriptions. From examination of all these forms, I find it now practicable to determine the true nature of the genus Solenopora, Dyb.