230 HAWAIIAN AND OTHER PACIFIC ECHINI. 



the ambulacral plates, are obviously made up of three elements, as in the Echini- 

 dae, an adoral primary element and two secondary elements above it. But every 

 fourth plate is typically greatly enlarged at its inner end and shuts out the three 

 succeeding plates from the median line. Examination of the exterior shows 

 that it is the primary tubercle of this enlarged plate which has grown over the 

 four plates concealing their sutures, and giving the impression of high ambulacral 

 plates with a dozen pairs of pores. So far as known this type of ambulacral 

 plate does not occur elsewhere among Echini. The abactinal system of Stomop- 

 neustes is much like that of. the Strongylocentrotidse; that is, oculars I and V 

 are typically in contact with the periproct, while ocular IV, or less commonly 

 ocular II, may also reach it. The periproct is covered by numerous small plates, 

 many of which carry spinelets. No suranal plate is distinguishable in mature 

 specimens. 



The spines of Stomopneustes are very stout and moderately long. In cross- 

 section they are obviously polycyclic. There are several (4-8) sphseridia on 

 the actinal part of each ambulacrum; each is attached to a miliary tubercle, 

 but is not associated with any depression or other shelter. The pedicellarise and 

 remarkable spicules have been fully described by other writers (see Mortensen, 

 1903, "Ingolf," Ech., pt. 1, p. 126). In a careful examination of four specimens 

 from the Persian Gulf, Mozambique, and New Guinea, only one kind of globif- 

 erous pedicellarise was found, the characteristic form, and I can therefore fully 

 confirm Mortensen's statement as to the scarcity of this to him very important 

 organ; for on the two largest and best preserved specimens, there appear to be 

 none, and on each of the small specimens but a single example. As Mortensen 

 gives no measurements it may be well to add some based on my observations. 

 The valves of the globiferous pedicellarite are about .80 mm. long, while the base 

 of each is about .35, and the tip of the blade .07 mm., in width. The tridentate 

 valves range from .30 to 1.10 mm. in length, the ophicephalous from .35 to .80, 

 besides the loop, and the triphyllous, which are the only really common pedicel- 

 larise, from .15 to .20 mm. The extraordinary spicules of the tube-feet are about 

 three quarters of a millimeter in length and about one tenth as much in diameter. 



Lack of material makes it impossible to decide positively whether there is 

 more than a single species in this genus. Specimens from New Guinea do 

 not seem to be distinguishable from those from Mauritius, but they are not 

 "almost black" and so may not be identical with the supposed form "atropur- 

 pureus" from Queensland. The examination of numerous specimens from a 

 considerable number of localities makes it very doubtful whether the characters 



