216 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



nearly or quite white. The largest specimen is 31 mm. h. d., while the primaries 

 are sometimes 75 mm. long. The abaetinal system is often elevated 3 or 4 mm. 

 above the test. The " Albatross " collected this species in 1891, in the deep 

 water between Acapnlco and Panama, and the Galapagos, 465-1880 fths., while 

 in 1904-05 she found it common in the still greater depths south and south- 

 west of the Galapagos, 2005-2153 fths. 



Aporocidaris fragilis. 

 Aporocidaris fragilis A. Agassiz and Clark, 1907, Haw. Pac. Ech. Cid., p. 37. 

 Plate 23, figs. 5-8, Haw. Pac. Ech. Cid., A. Agassiz and Clark, 1907. 



Of this species, the only known specimens, the largest of which is 23 mm. h. d., 

 with primaries over 40 mm. long, were taken by the " Albatross " in the North 

 Pacific, south of Alaska and southwest of Kamchatka in 1557-1973 fths. 



Aporocidaris incerta. 



Porocidaris incerta Koehler, 1902, Belgica Ech. et Oph., p. 7. 



Figs. 2, 16, Belgica Ech., Koehler, 1903. 



Koehler's supposition that this species is related to milleri is quite correct, 

 though in the shape of the test it is more like fragilis. The position of incerta 

 in this genus is confirmed not only by Mortensen's (:03) examination of the pedi- 

 cellariae, but by a careful comparison of Koehler's description, with a specimen 

 of fragilis of the same size (15 mm. h. d.) as his largest specimen. It is difficult 

 to make out from that description just how much difference there is between the 

 Arctic and Antarctic species. The latter was taken by the " Belgica " about 

 20 degrees south of Kergnelen Island, in 55-165 fths. 



Stereocidaris. 



Stereocidaris Pomel, 1883, Class. Meth. Gen. Ech., p. 110. 



Test very similar to Dorocidaris, but usually flatter (.50-. 60 h. d.), with fewer 

 coronal plates (4-7, rarely 8 or 9) and relatively fewer primary spines (3-7, rarely 

 8, in each vertical series) ; that is to say, uppermost coronal plate without primary 

 spine, and second often, third very rarely, similarly bare. Abaetinal system large 

 (.35-55 h.d., usually about .50), often convex, and noticeably thick and stout, but 

 this character varies much within a single species ; abaetinal miliaries and second- 

 aries usually very small, but this character also varies much. Primary spines 

 usually flaring at tip, or if tapering, provided at base with conspicuous buttress-like 

 "wings"; " winged " primaries are usually noticeably compressed, but otherwise 

 primaries are cylindrical. Globiferous pedicellariae, large and small, commonly 

 lack a conspicuous end-tooth on valves. 



This is the most poorly defined and unsatisfactory genus in the family, and 

 yet the species contained in it have something about their general appearance 

 whicli is distinctive and makes it possible to recognize them usually at a glance. 

 They show considerable diversity in test, spines, and pedicellariae, and some in- 

 dividuals are strikingly like Dorocidaris. It is only when a considerable amount 



