KUNGL. SV. VET. AKADEMIENS HANDL. BAND. 19. N:0 7, 91 



lar, is posterior, somewhat sub-veiitral; the excretory opening occupies the centre of tlie 

 anal nieiiil)rane which is covered with about five concentric rows of scales. 



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

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

 mula is normal. In tlie bivium the plates 1 a 2, 3, 4, I b S, 4, 5, and V b 2, 3, 4, V a 

 3, 4, 3, are lengthened, and the lad and \' b 4 slightly expanded interiorly, so as 

 to fill up the feeble re-entering angle offered b}' the corresponding plates of the po- 

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

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

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

 present in all Prymnodesmic Spatangidte. Its deficiency in Urechinus is a sure sign 

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

 fully examined showed the least trace. There is, close under the periproct, a dense 

 accumulation of ordinary miliary tubercles, not unlike 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 3, are very much alike, u]) to !l 

 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. 

 Tlie minute pore, from !) or 10, becomes more distant from the adoral margin, gradu- 

 ally Hearing 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 6 5 of 2 and 3 being, as 

 observed hitherto in Palasostoma mii'abile alone, united into single plates, and like- 

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

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

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

 rity, a feature that seemed to approximate it to the Cassidulida3. In the Holastrida% 

 on the other hand, in Holaster, Anancites, Off'aster '), Cai'diaster, the heteronomy, ren- 

 dered by the formula 1 a 2 -{- b 2 — 4 a 2 : 4 b 2 '^), \s distinctly seen in well 

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

 neustes striatus (!m. ') several specimens were subjected to repeated and careful scru- 

 tiny without yielding decisive evidence of the presumable heteronomy, 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 Spatangidie, the 

 Meridostcrni, among which the asymmetry is still unsettled and, as it were, in its be- 

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

 assumes in the Prymnadetes and the more recent Prymnodcsmians. 



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

 contiguous to I a 1, 2 and V b 1, 2. The plate 5 b 2 simulates, as it wei'e, by itself 



') See the woodcut ou the next page. 

 '^) See above p. 15. 

 ^) See the wood-cut. 



