The animals either wholly new, or new to Britain, described in this communication, were taken during a yachting cruise with Mr Macandrew, of Liverpool, among the Hebrides, in the month of August 1850. During this voyage, which lasted three weeks, a series of observations were conducted by means of the dredge and towing-net. Not a single new testaceous Mollusk was procured; but several remarkable Ascidians and Radiata were discovered, some of them so curious in themselves, and so important in their zoological bearings, that the authors of this paper thought it desirable to lay an account of them before the Royal Society of Edinburgh.