I beg leave to lay before the Royal Society a few observations on the genus Planaria, to which I have been more immediately led, from two or three species having been already described in the Linnæan Transactions under the genus Hirudo. I shall confine my remarks in the present paper to the following species: P. cornuta , P. torva , and P. brunnea , P. lactea , intending to give, at some future period, the farther history of this singular and extensive genus.