by Ehrenberg only by the short diagnosis, "Radiis stellæ tribus apice truncatis." It is probably identical with his figure of Dictyastrum angulatum (loc. cit.). This latter name I have retained for the similar Porodiscid (above, p. 526), mainly because the genera Dictyastrum and Rhopalodictyum, according to the insufficient diagnosis of Ehrenberg, seem to be identical. (Compare my Monograph, 1862, p. 466.)
Dimensions.—Radius of each arm 0.13, basal breadth 0.09, distal breadth 0.1.
Habitat.—Tropical Pacific, Philippine Sea, Station 200, depth 250 fathoms; Atlantic (Mexican Gulf Stream).
3. Rhopalodictyum subacutum, Ehrenberg.
Arms of equal size and equidistant, club-shaped, three times as long as the diameter of the central disk, which equals the breadth of the thickened distal end; the latter is armed with a strong pyramidal terminal spine. (The diagnosis of Ehrenberg is "Radiis stellæ tribus, apice cuneatis subacutis.")
Dimensions.—Radius of the arms 0.2, basal breadth 0.04, distal breadth 0.07.
Habitat.—North Atlantic, surface and various depths; Greenland; Mexican Gulf Stream; Færöe Channel (John Murray).
4. Rhopalodictyum bifidum, n. sp.
Arms of equal size and equidistant, in the distal half forked, twice as long as the diameter of the central disk; both fork branches half as broad as the simple basal part, truncated at the distal end.
Dimensions.—Radius of the arms 0.16, basal breadth 0.08, distal breadth 0.04.
Habitat.—North Pacific, Station 237, surface.
Subgenus 2. Triactinosphæra, Dunikowski, 1882, Denkschr. d. k. Akad. d.
Wiss. Wien, vol. 45, ii. p. 192.
Definition.—Triangular shell bilateral or irregular, with three arms of different size or distance.
5. Rhopalodictyum zittelii, Haeckel.
Arms of different size and at unequal distances, one odd arm being a little shorter than the two paired arms; the odd angle between the latter is larger than the paired angles between them and the
