52 VERRILL 



together in young specimens. Upper and lower marginal plates 

 wide and strong, each bearing, in the young, a single spine. The 

 upper marginal plates have a large and wide descending lobe ; these 

 join the stout upper lobe of the inferomarginals, but leave wide 

 papular areas between them, along the wide lateral channels. The 

 upper marginals have small side lobes, which are united together 

 longitudinally, by smaller narrow, interpolated spineless plates. 



Peractinal plates are small, or rudimentary without spines. The 

 adambulacrals generally appear to be joined directly to the infero- 

 marginals, but minute ossicles may intervene. Adambulacral plates 

 diplacanthid or subdiplacanthid. (PI. lxx, fig. i.) 



Major pedicellariae are large and numerous, especially between the 

 inferomarginal spines and on the inner margins of the grooves, but 

 also occur on the back. They are acute-ovate and a little com- 

 pressed. The minor pedicellariae are unusually large and denticu- 

 late; they form very large pedicellate clusters, especially on the 

 inferomarginal spines. Ambulacral pores and feet very large. 



The lack of distinct peractinal plates and spines and the presence of 

 the large interradial plates are very important morphological char- 

 acters, aside from the feeble dorsal skeleton. The large size of the 

 ambulacral pores is a notable feature. 



The type, U. linckii := stellionura (Per.), is found from Spitz- 

 bergen to Nova Scotia.* Another arctic form, U. gunneri (Diib. and 

 Kor.), is closely allied and may be only a variety (teste Ludwig). 

 One or both of these may very likely be found hereafter in Bering 

 Sea, for they are probably circumpolar. 



Another arctic species, U. panopla (Stuxberg) seems to be con- 

 generic with U. linckii. It has a similar, very feeble, reticulated dorsal 

 skeleton, and lacks distinct peractinal plates and spines. Its proximal 

 adambulacral plates in some cases bear three or four spines, but are 

 mostly diplacanthid. The superomarginal and inferomarginal plates 

 are cruciform and are connected by a narrow, spineless, intermediate 

 plate. The inferomarginal ossicles usually bear two spines, some- 

 times more in large examples. It is from Spitzbergen, Barents Sea, 

 Kara Sea, Finmark, etc. It is remarkable for the great number 

 and large size of the dorsal minor pedicellariae. 



* It has been found as a fossil abundant in the postglacial clays at Salem, 

 Massachusetts. 



