KONGL. SV. VKT. AKAIJKRUEN.S 1IAN]M>. BANIJ. 19. N:0 7, 91 



lar, is postcrioi', .soniuwhat sub-veiitral; the excretory openiiig occupics tlic ceiitrc; of tlie 

 ajial iijem))ra.uc whicli is covered with about live coiicentric rows of scales. 



The ambulacral system is normal and closely resembling that of Anancites by 

 its lanccolate, nearly uniform radii, all level with the perisome. The peristomal for- 

 mula is normal. In the bivium the plates I a 2, 3, 4, I b 3, 4, 5, and Y b 2, 3, 4, V a 

 3, 4, 5, are lengthened, and the I a 4 and \ b 4 slightly expanded interioi'ly, so as 

 to iill up the feeble re-entering angle offered by the corresponding plates of the po- 

 sterior interradium, a structure commonly met with also in Holaster and other Merido- 

 sterni, and in the Prymnadetes, that is, in forms devoid of a subanal fasciola, and in 

 no wise to be compared with the Avell-known wedge-shaped, extended plates 6 + x, 

 present in all Prymnodesmic Spatangida3. Its defieiency in Ui-echinus is a sure sign 

 of the absence of a subanal fasciola, of Avhich not one of the several specimens care- 

 fully examined showed the least trace. There is, elose under the periproet, a dense 

 accumulation of ordinary miliary tubercles, not un like that seen in the same position 

 in some Brissi; it has no relation to the fasciola. 



In all the five ambulacra, the plates, from 4 or 5, are very much alike, up to O 

 at least twice as broad as they are long, then longer in proportion to the breadth, 

 and finally, from 12 or 13, approaching to a somewhat equilateral hexagonal form. 

 The minute pore, from B or 10, becomes more distant from the adoral margin, gradu- 

 ally nearing the centre. Thus all the ambulacra are apetalous-. 



The perisome presents the almost exceptional peculiarity of having the paired 

 interradia perfectly symmetrical, the two plates a 2 and b 2 of 2 and 3 being, as 

 observed hitherto in Pala3ostoma mirabile alone, united into single plates, and like- 

 wise the plates a 2 and b 2 oi 1 and 4. By this last unique mode of coalescence 

 these interradia have become symmetrical towards one another, and the heterononiy 

 of 1 is discarded. Hitherto Collyrites apparently was alone in presenting this regula- 

 rity, a feature that seemed to approximate it to the Cassidulidse. In the Holastrida^, 

 on the other hand, in Holaster, Anancites, Offaster ^), Cardiaster, the heterononiy, ren- 

 dered by the formula 1 a 2 -\- b 2 = 4 a 2:4 b 2^j, is distinctly seen in well 

 preserved specimens, though in. others it may be difticult to make out. Of Hemip- 

 neustes striatus Gm. ^) several specimens were subjected to repeated and careful scru- 

 tiny without yielding decisive evidence of the presuraable heteronoray, and now, being 

 convinced of the perfect regularity of the corresponding parts in Urechinus, I venture, 

 though always with some doubt, to give the figure, on the next page, of that species, as 

 another example of perisomal symmetry in that early group of the Spatangida?, the 

 Meridosterni, among which the asymmetry is still unsettled and, as it were, in its bc- 

 ginning, and far from presenting the definite and constant character it gradually 

 assumes in the Prymnadetes and the more recent Prymnodesmians. 



The odd interradium, 5, is rather narrow; the labrum, expanded aborally, is 

 contiguous to I a i, :? and Y b 1, 2. The plate 5 b 2 simulates, as it were, by itself 



') See the woodcut on the next page. 

 -) See above p. 15. 

 ■*) See the wood-cut. 



