10 



form'). Besides D. saxatile (setosnm), he maintains only the West-American form, D. 

 mexicanum, as a distinct species, though the différences in test and spines, which are 

 pointed out as distinguishing cliaracters (Rev. of Ech. p. 408), are, indeed, vary slight. 

 But as „the pedicellariæ do not help us in the comparison", as Agassi/, states 

 (loc. cit.), there seems to be nothing else left, and all the later authors, indeed, follow 

 Agassiz in his view of the species of Diadema. 



In spite of this universal agreement I am opposed to the representation 

 of the üiadema-spec'ies given by Agassiz. I willingh' grant that only very slight 

 differences, scarcely sufficient to distinguish the species thereby, are found in test 

 and spines; but in the positive assertion that the pedicellariæ do not help us in 

 the discrimination of the species lies the fault. The pedicellariæ do indeed give 

 excellent specific characters. A closer examination shows that Ü. untillunim is very 

 well distinguished from D. saxatile, and that it is even more losely related to D. 

 mexicanum than to the former species. Further it becomes evident from an exami- 

 nation of the pedicellariæ that D. Savignyi Mich, also, which has likewise been 

 made a synonym of D. saxatile, is a distinct species, and I should not be surprised, 

 if there eventually proved to be yet more well distinguished species of this genus. 



The test of Ü. saxatile does not differ so much from tiie test of the other species 

 that it can always be recognised with certainty by the characters found in it alone. 

 The following features must be pointed out as more important distinguishing 

 characters. The pore-areas are somewhat widened towards the actinostome, the 

 actinal pores are much smaller than those above the ambitus, though their „peri- 

 podia" are comparatively much larger. They are arranged in close transverse series 

 (arcs) of three, and there is a primary tubercle only to about every three or four 

 arcs of pores. Above this widened part of the pore areas there is a primary 

 tubercle on each compound ambulacral plate — as in all the species of Diadema 

 know'ii to me. In a small specimen (35 mm. in diameter) this widening of the pore 

 areas is as yet only slightly developed, in large specimens it is very conspicuous. 



The naked part in the interambulacral areas is comparatively larger in this 

 species than in the others; the uppermost one of the large tubercles forming the 

 inner row is found on the fifth or sixth plate from above (beginning with the 

 first complete one), and these tubercles are thus scarcely seen above the ambitus. At 

 the lower end of the naked space there is a conspicuous white, angular spot, covering 

 the inner end of the three last naked plates. Also on the living animal this is seen 

 as a white spot. — The genital plates have mostly a distinct dark impression above 

 the genital pore (evidently corresponding to the blue spot seen there in the living 

 animal) (Fig. 1). This is, however, not an absolutely reliable character, as the 



') BÖLSCHE (Op. cit.) lias first maintained the Atlantic form to be the same species as the Indopacific 

 form. V. Martens (Op. cit. p. 156) thinks that improbable, but since Agassi/, in his Revision declared 

 himself to be unable to distinguish the Atlantic from the Indopacific form, nobody lias doubted the 

 correctness of this view. 



