THE nomenclature of some of the species of Potamogeton is difficult to deal with satisfactorily; I propose here to discuss P. tenuifolius. Three distinct plants have been so named:-P. tenuifolius Rafinesque, Med. Rep. 409 (1311); P. tenuifolius H. B. K. Nov. Gen. et Sp. i. 297 (1815); P. tenuifolius Philippi, ined., in herb. Berlin, et in litt. ! (1864).

Of the first of these we have no definite information; the only suggestion one can make is that it was either a form of P. alpinus Balbis, or of P. heterophyllus Schreb. But should it prove something different from these, the name must stand.

P. tenuifolius H. B. K. must be referred to P. pectinatus L. as a variety.

The doubt about Rafinesque's plant renders it inadmissible to adopt Philippi's MS. name, although the plant to which he gives it is a good species. I therefore propose to name it P. Aschersonii,* after Prof. Ascherson, who has added so much to the knowledge of the marine genera of the order.

Its affinity is with P. pusillus L. and P. Berteroanus Philippi in Linnæa, xxx. 200 (1859-60). It varies considerably in its leaves, but so many of the South American specimens of the pusillus group are badly preserved, that without "soaking out" it is difficult to determine their true character.

I refer to it the following gatherings:-Chili, Philippi! Valparaiso, Bridges! Argentina, Hieronymus! Tweedie!? Uruguay, Gibert Columbia, Jameson! Brazil, Gardner! Prov. Rio de Janeiro, Glaziou!

P. Aschersonii mihi. P. tenuifolius Philippi ined. Stems slender, terete, 6-18 in. long, branched, especially in the lower half; rarely with long (3-4 in.) internodes. Leaves variable,

* I have ascertained from Prof. Ascherson that he has not named the plant, as I thought he might have done, for the Fl. Brasiliensis.

linear, 1-3 in. long, 1-1 in. broad, 3-veined; the outer slender veins connected with the midrib by very fine irregular cross-veins; subacute. Stipules soon decaying, 6-9 lines long, yellowish white, those enclosing the peduncles more persistent and broader. Peduncles slender, equal, 1-2 in. long; spikes 4-6 lines long, with 6-9 fruits. Sepals ovate, with a rounded base. Fruit 21 lines by 11 in. broad, ovate (or slightly obovate), nearly flat on the sides, and impressed with a shallow depression; the 3 keels sharply defined by raised lines on the smooth surface of the ventral face of the fruit, and without any tubercules; beak prominent on the dorsal side of the medial line. Embryo-apex nearly touching the basal end.

P. SPIRILLUS Tuckerman in Sill. Journal, 2nd series, vol. vi. p. 228 (1848). Dr. Morong, in his Mon. Fl. American Naiad., queries my reference of P. Zetterstedtii Wallman (Schl. & Mohl. Bot. Zeit. i. 256 (1843), as belonging to the above plant. While fully believing it does so, I cannot say I have seen a specimen to prove it. But it is of secondary importance, if I am right in believing that Tuckerman's plant must bear the name of P. dimorphum Rafinesque in Month. Mag. & Critical Review, p. 358 (1817).

Barton (Fl. Philad. Prod. (1815)) names a new species P. diversifolius (it is doubtful whether he knew of the publication of Rafinesque in 1808 of the same name), and says it is distinct from P. hybridus Michx. (1803). In 1823, in his Flora of N. America, t. 84, vol. iii., he figured his species, and the plate seems to me to represent P. spirillus, if there is any difference between that and P. hybridus as species. Rafinesque, reviewing Barton's Flora (1815) in Monthly Mag. & Critical Review of 1817, remarks that his (Barton's) plant is different from his diversifolius, and hence from hybridus of Michx., and preferred the name P. dimorphum for it; and it seems to me that it must bear that name, and that Tuckerman's becomes a synonym.

As Dr. Morong uses P. diversifolius Raf. for P. hybridus Michx., because the latter had been used by Thuillier (or rather Pentagna) for P. heterophyllus Schreb., it follows that Barton's diversifolius will become a synonym of P. dimorphum Raf.

But the "law" that is desired to be forced on us, "that any species or variety that has been so named, under any other species or variety, cannot be used in the same genus," will be of somewhat difficult application. Students certainly will never know, and even monographers will not be safe, as proved by Dr. Morong's own work, where he must (by his own law) change the names of at least three of his species, having failed to ascertain that they were in use before.

Most of the American authors (Gray, &c.) refer Barton's diversifolius to P. hybridus Michx.; but I do not see how this could have been done with Barton's plate in existence, and his and Rafinesque's positive declaration to the contrary. These facts cannot be put aside by any suggestion of looseness of naming, &c.

P. FLUITANS Roth, Fl. Germ. i. p. 72 (1788): ii. p. 202. In his recently published Monograph, Dr. Morong remarks that he hesitates to identify P. Lonchites of Tuckerman with the plant usually