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Show 410 PROF. J. A. THOMSON AND MR. W. D. HENDERSON ON [Apr. 10, Order [\. A LCYONACEA, Verrill {pro parte). Family XENIID^. Xenia umbellata Savigny. ,, quinqueserta May. ,, membranacea Schenk. ,, cozrulea Ehrenberg. ,, ternatana Schenk, var. elongata,, nov. ,, rigida, sp. n. Heteroxenia elisabethce Kolliker. Cespitularia coerulea May. Note on the Sp)ecies of Xenia. In the genus Xenia, -as in the genus Glavularia, the question of species is a difficult one. All the species are within a relatively narrow range, and the differentiating characters are, when taken separately, somewhat trivial. Even in the same colony there are sometimes noteworthy differences in the adjacent polyps-differences which are sometimes as marked as any one of the separate items which are used to distinguish species. It seems likely that many of the differences are purely modificatioual, and referable to differences in nutrition and the like. The number of rows of pinnules is a character which has been much relied upon, but it is apt to lead one astray unless the tentacles observed are equally extended. Moreover, there may be 3 rows at the proximal end and 4 about halfway up, or 2 at the proximal end and 3 about halfway up the tentacle. There can be no confusion between a species with one row of pinnules on each side and a species with four rows on each side, but to distinguish two species because one has three rows and the other four appears to us quite misleading unless this detail is supplemented by many others. In X. umbellata we found from 2-4 rows, in X. quinqueserta 3-5 rows. Some workers have attached importance to the presence of wartlike pinnules along with others of the usual elongated type ; but the presence of a few wart-like pinnules at the proximal end .appears to us to be very common, and may be naturally expected when a polyp is not fully grown,. At the same time, the minute warts of, for instance, A", rigida are diagnostic in contrast to the long pinnules of X. umbellata. Another diagnostic feature is the presence or absence of a bare strip on the surface or surfaces of the tentacle, but this is apt to be obscured or exaggerated by the degree of contraction. The bare streak may be present at the proximal end and absent higher up, or quite distinct along the distal half and quite obscured by contraction lower down. XENIA UMBELLATA Savigny. The length of the stalk is 24*5 mm., with a maximum basal |