( 323 ) XVIII. A Synopsis of the Testaceous Pneumonobranchous Mol-lusca of Great Britain. By J. G. Jeffreys, Esq. In a Letter addressed to L. W. Dillwyn, Esq. F.R. and L.S. Read November 18, and December 16, 1828. Dear Sir, If you think that the following account of an interesting branch of our native Testacea, compiled from my own observations and a careful investigation of those authors who have written on the subject, will be found worthy the notice of the Linnean Society, I shall be glad to avail myself, with permission, of your medium in submitting it to that learned body. The few changes in sy-stem and nomenclature which I have ventured to propose are very partial, and I am sure that you will think with me that they are justifiable in an attempt to ascertain uncertain characters, or determine new ones. I do not lay much stress upon the distinct-ive marks furnished by the animals of this order specifically con-sidered, though I am fully aware of their importance in forming the leading features of the higher divisions. The accompanying catalogue will I believe be found to comprise all the species hitherto known as British, with some not inconsiderable ad-ditions. They are, with a single exception, in my Cabinet. I am, Dear Sir, Yours truly, J. G. Jeffreys. Swansea, September Ist, 1828.