708 ME. T. T. GKOOH ON SOME 



a modification of the oligoporous Echinoid type, in which the two 

 aboral primaries are represented by demiplates * . 



They could not, however, arise from the fusion of three of the 

 oligoporous plates of the Echinothuridae f, where we also meet 

 with a trigeminal arrangement, as there the demiplates seem to be 

 adorally placed. 



The ambulacral areas seem at first sight to present a considerable 

 amount of resemblance to those of Astropyga pulvinata +. This form 

 has oligoporous plates, each of which has a tubercle, every third 

 plate being larger than the other two, this giving rise to an S-shaped 

 median line. Prof. Duncan has, however, shown that the triplets 

 are arranged after the type of Diadema§. 



Eig. 4. — One of the Hows of Imbricating Actinal Plates in Pelane- 

 chinus corallinus, with the adjacent interambulacral (i) and 

 ambulacral (a) areas. X 3. 



p. Peristome. 



The Imbricating Peristomial Plates (fig. 4). — These, in my specimen, 

 are not well shown ; but the results of a re-examination of a specimen 



* Duncan, loc. cit. 



t It is usually stated that the plates of the EchinothuridaB are simple pri- 

 maries, but the figures of Agassiz (' Challenger ' Report) make it clear that they 

 are, at any rate in many cases, oligoporous, the demiplates ( = the so-called acces- 

 sory plates) which seem constantly to occur on the adoral side of the primary 

 being distinctly fused with the latter. See figs, in ' Challenger ' Report : — 

 Phormosoma hoplacantha (pi. xiia. fig. 12), Asthenosoma tessellata (pl.xxia.fig. 14), 

 Asthenosoma Grubei (pi. xvii. fig. 6), Asthenosoma pellucida (pi. xviii. fig. 5). 



\ Journ. Linn. Soc, Zool. vol. xix. p. 95, pi. v. fig. 9. 



§ Duncan, Journ. Linn. Soc, Zool. vol. xix. p. 106. 



