528 PROCEEDINGS OF THE AMERICAN ACADEMY 



The genera Ilorkelia, Cliam. &. Sclilecht., Icesia,ToYr. k. Gray, and 

 Sibbaldia, L., border so closely upon Potentilla that, for lack of abso- 

 lute distinctions, they are not unlikely all to be referred to that already 

 vast genus.* Such a combination, however, will hardly be thought sat- 

 isfactory, even if unavoidable. The Linnaian genus, Sibbaldia, appears 

 to have the least claims to its rank, having no peculiarity of habit, and 

 comprising pinnate as well as trifoliolate species, which may well be 

 regarded as Potentillas with the stamens, and in a less degree the car- 

 pels, reduced in number. If kept distinct, the deeply lateral styles 

 will aid in separating Sibbaldia from the following. 



Horkelia, however, if strictly limited, is pretty well marked in habit, 

 and decidedly so in technical characters. These are the campanulate 

 calyx and the (10) dilated filaments. This last and most distinguishing 

 feature was omitted from the original generic character, and also from 

 that of Endlicher. 



Ivesia (striking out T. gracilis) I think should also be kept distinct 

 from Potentilla, and probably from Horkelia. It cannot fail to be re- 

 tained as a genus, if Sibbaldia is. There are now several species, all 

 with the same peculiarity of foliage, which is quite unlike anything in 

 Potentilla. Its characters are the campanulate or cyathiform calyx, 

 the definite (5 to 20) stamens with slender filaments, and the few, in 

 one instance even solitary, carpels. 



In thus arranging these plants, it will be seen that there is one am- 

 biguous and very troublesome species, viz. Horkelia tridentata, Torr., 

 which, with the calyx and nearly the habit of Ilorkelia, has slender 

 filaments. They are not, however, filiform, nor inserted, with the pet- 

 als, at the bottom of the calyx, as represented in the plate. They are 

 short, and inserted in the throat of the calyx, as in Ilorkelia, from 

 which they differ only in being subulate. My specimen is insufficient. 



Horkelia, Cham. & Schlecht. 



Calyx campanulatus, 10-fidus. Petala unguiculata seu spathulata 

 (alba vel rosea). Stamina 10, fauci calycis inserta : filamenta brevia, 

 dilatata, plerumque deltoidea petaloidea persistentia. Carpella pluri- 

 ma, nunc pauciuscula, in receptaculo villoso parvo elevato insidentia : 



* This has been done in the vet unpublished second part of Bentham and 

 Hooker's Genera Flantarum. 



