SIR,--If my recollection does not deceive me, the collection of the remains of the animal incognitum of the Ohio (sometimes called mammoth), possessed by the Cabinet of Natural History at Paris, is not very copious. Under this impression, and presuming that this Cabinet is allied to the National Institute, to which I am desirous of rendering some service, I have lately availed myself of an opportunity of collecting some of those remains. General Clarke (the companion of Governor Lewis in his expedition to the Pacific Ocean) being, on a late journey, to pass by the Big-bone Lick of the Ohio, was kind enough to undertake to employ for me a number of laborers, and to direct their operations in digging for these bones at this important deposit of them. The result of these researches will appear in the enclosed catalogue of specimens which I am now able to place at the disposal of the National Institute. An aviso being to leave this place for some port of France on public service, I deliver the packages to Captain Haley, to be deposited with the Consul of the United States, at whatever port he may land. They are addressed to Mr. Warden of our legation at Paris, for the National Institute, and he will have the honor of delivering them. To these I have added the horns of an animal called by the natives the Mountain Ram, resembling the sheep by his head, but more nearly the deer in his other parts; as also the skin of another animal, resembling the sheep by his fleece but the goat in his other parts. This is called by the natives the Fleecy Goat, or in the style of the natural historian, the Pokotragos. I suspect it to be nearly related to the Pacos, and were we to group the fleecy animals together, it would stand perhaps with the Vigogne, Pacos, and Sheep. The Mountain Ram was found in abundance by Messrs. Lewis and Clarke on their western tour, and was frequently an article of food for their party, and esteemed more delicate than the deer. The Fleecy Goat they did not see, but procured two skins from the Indians, of which this is one. Their description will be given in the work of Governor Lewis, the journal and geographical part of which may be soon expected from the press; but the parts relating to the plants and animals observed in his tour, will be delayed by the engravings. In the meantime, the plants of which he brought seeds, have been very successfully raised in the botanical garden of Mr. Hamilton of the Woodlands, and by Mr. McMahon, a gardener of Philadelphia; and on the whole, it is with pleasure I can assure you that the addition to our knowledge in every department, resulting from this tour of Messrs. Lewis and Clarke, has entirely fulfilled my expectations in setting it on foot, and that the world will find that those travellers have well earned its favor. I will take care that the Institute as well as yourself shall receive Governor Lewis's work as it appears.