In order to collect the petrifactions, and to render them useful, it is not necessary to know them, nor to be perplexed to find them out; nor to be afraid of sending objects already known or of little note. A part of the preceding indications, connected with the most common petrifactions, will always render them useful. The important point then is, not to mix those which are found separate, nor to separate those which are found associated in the same stratum. This is easily attained, by designating by a common number, letter, or any sign whatever, one particular formation or stratum, and by marking with the same sign all the petrifactions which are evidently found together. The labels designating the place or the geological situation, may be placed in the papers which envelope the specimens, or a number, referring to an explanatory catalogue, may be attached to each specimen. As far as possible, it is necessary to stick the labels or numbers to the pieces, by pasting; and the surest way is, to write upon the piece itself, 1st, the place where it is found; 2d, the number by which it is indicated in the historical notes above requested. If there is not time to make out as many numbers or labels as there are pieces, it will be sufficient to unite in one box or packet all the petrifactions of one particular stratum, and to designate them by a general label. It is necessary to pack the shells and other fragile pieces in separate boxes, and to wrap each piece in a separate paper. M. Brongniart cannot allow himself to prefer such requests, except under the express condition, that a memorandum of all the expenses which the transportation and packing of the specimens may create shall accompany the letter of advice. The objects destined for him may be sent by the common modes of conveyance, with a letter of advice, to the following address: Mr. A. BRONGNIART, Member of the Royal Academy of Sciences, Engineer of Mines, etc. Rue Saint-Dominique, Faubourg Saint-Germain, No. 71, Paris. ART. XIV. _Observations on a species of Limosella, recently discovered in the United States, by Dr. Eli Ives, Professor of Materia Medica and Botany, in the Medical Institution of Yale College._ This small plant was observed in flower in July, 1816, by Mr. Horatio N. Fenn (now of Rochester, State of New-York) in company with Dr. Leavenworth. The plant and the seeds have been preserved by me, in a flower-pot, from that time to the present. The plant was taken a few rods south of Mr. Whitney's gun manufactory, on the margin of the river, where it was covered by every tide. I have since observed the plant in great abundance on the margin of the Housatonuck, in Derby, and in those small streams in East Haven, Branford, and Guilford, which empty into Long-Island Sound.