590 THE VOYAGE OF H.M.S. CHALLENGEE, 



by Ehrenberg only by the short diagnosis, " Eadiis stellaj tribus apice truncatis." It is 

 probably identical with his figure of Didyastrum angulatum (loc. cit.). This latter name I have 

 retained for the similar Porodiscid (above, p. 526), mainly because the genera Didyastrum and 

 Bhoimlodidyum, according to the insufficient diagnosis of Ehrenberg, seem to be identical. 

 (Compare my Monograph, 1862, p. 466.) 



Biinensions. — Eadius of each arm OlS, basal breadth 0'09, distal breadth O'l. 



Habitat. — Tropical Pacific, Philippine Sea, Station 200, depth 250 fathoms ; Atlantic (Mexican 

 Gulf Stream). 



3. Rliopcdodictyum suhacutum, Ehrenberg. 



Ehojjalodidijicm suhacutum, Ehrenberg, 1861, Monatsber. d. k. preuss. AkaJ. d/ Wiss. Berlin, 

 p. 301. 



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 " Eadiis stella:" tribus, apice cuneatis 

 subacutis.") 



Dimensions. — Eadius of the arms 0'2, basal breadth 0'04, distal breadth 0'07. 



Ealitat. — North Atlantic, surface and various depths ; Greenland ; Mexican Gulf Stream ; 

 Fa3roe 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. — Eadius of the arms 016, basal breadth 0"08, distal breadth 0'04. 

 Habitat. — North Pacific, Station 237, surface. 



Subgenus 2. Triactinosphwra, Dunikowski, 1882, Denkschr. d. k. Akad. d. 

 Wiss. Wieu, vol. 45, ii. p. 192. 



Definition. — Triangular shell bilateral or ii-regular, wth three arms of different size 

 or distance. 



5. Rhopalodictyum zittelii, Haeckel. 



TriadinosphcBrazittelii, Dimikowski, 1882, Denkschr. d. k. Akad. d. Wiss. Wien, vol. 45, ii. p. 192. 



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 



