102 



species, whose extreme forms; Microcyphus macnlatus and Ambhjpneustes pallidiis 

 (or any other Amblypneustesspecies) differ so much that it would he impossible to 

 unite them in the same genus. 



The genera Amhlypneustes and Holopneustes are notorious for their difficulty. 

 A rich material will be needed for definitely working out the rather numerous 

 species; such is, however, not yet found in the collections of the different Museums. 

 By far the largest number of the preserved specimens are naked tests, which do 

 not afford sufficient characters for the distinction of the species. Especially it is an 

 unfortunate circumstance that the type-specimens are naked tests, if they are found 

 at all, which is not the case with all of them. As I have examined the specimens pre- 

 served in the British Museum and the Paris-Museum, and as the Copenhagen-Museum 

 is comparatively rich in these forms, I am able, in spite of the unfortunate circum- 

 stances pointed out above, to make some remarks on this subject, which, I think, 

 will not prove valueless. 



First an important character may be pointed out, by which the genera 

 Amhlypneustes and Holopneustes are easily distinguished, viz. that in all the Ambly- 

 neus/es-species (except A. pentagonus — see below) there is a primary tubercle on all 

 the ambulacral plates; in the //oZojD;ieus/es-species a primary tubercle is found only 

 on every second or third ambulacral plate, or even more irregularly. By this 

 character even quite small specimens, in which the pores are placed in regular 

 arcs of three as in Amhlypneustes, may be safely distinguished as belonging to 

 Holopneustes; in a specimen of 10 mm. diameter I find the alternation of the primary 

 tubercles quite distinct. — It may be remarked that in large specimens of Holo- 

 pneustes several of the ambulacral plates do not reach the median line of the 

 ambulacral area, thus looking like small primaries. In reality they are compound 

 plates; the number of tube feet has been so exaggerated that the plates have become 

 very low and almost rudimentary to make room for all the feet (Comp. „Ingolf"- 

 Echinoidea. I. p. 132-3). Gregory') has pointed out the same feature in Tripneustes, 

 remarking that, in fact, here are more than two series of ambulacral plates, as 

 in the Palæechinoiclea. This feature is much more distinct in Holopneustes. Of 

 course it is really something very different from the pluriseriate ambulacra of the 

 Palæechinids, as in the latter all the ambulacral plates are primaries, whereas in 

 Tripneustes and Holopneustes they are very modified compound plates. 



Among the species referred to Amhlypneustes one has already been removed 

 from that genus by Duncan (Revision p. 113) and made the type of a separate genus, 

 Goniopneustes. (Agassiz himself supposes („Rev. of Ech." p. 483) that it will eventu- 

 ally form a separate genus between Salmacis and Amhlypneustes.) To the characters 

 pointed out by Agassiz and Duncan I may add one, which is seen in the figures 

 (photographs) of the species given by Agassiz (Rev. of Ech." PI. VIII. c. 7—8) viz. 

 that only every second or third ambulacral plate has a primary tubercle. Pedicellariæ 

 ') In Rav Lankestebs Treatise oa Zoology. HI. 1900, p. 299. 



