KErOET ON THE ASTEROIDEA. 173 



3. Ctenodiscus procurator, n. sp. (PL XXX. figs. 7-12). 



This form has so many points of close resemblance to the North-Atlantic Ctenodiscus 

 corniculatus that examples might be selected which at first sight would easily be 

 mistaken for that species. A number of small differences, however, present themselves 

 when a large series is examined, which appear sufficiently constant to warrant the recog- 

 nition of this form as a distinct species. Under these circumstances the description of 

 Ctenodiscus procurator will probably be most intelligible if it takes the form of a com- 

 parative review of the characters of this species in relation to those of the two previously 

 known species of Ctenodiscus, viz., Ctenodiscus corniculatus of the North Atlantic, and 

 Ctenodiscus autralis, Liitken, from the East of Patagonia. 



When these three species are compared inter se it is evident that in many respects 

 Ctenodiscus corniculatus, though so widely separated geographically, appears to occupy 

 an intermediate classificatory position between Ctenodiscus australis and Ctenodiscus 

 procurator, which inhabit the eastern and western sides respectively of South America. 

 In Ctenodiscus procurator the rays are generally a trifle longer, and, even when not 

 actually so, have at least that appearance in consecpience of being slightly narrower at 

 the base and more attenuate and pointed outwardly. The abactiual area is plane, its 

 union with the lateral wall, especially in the region of the disk and the base of the rays, 

 forming a sharp angle in consequence of the rapid adoral slope of the whole lateral wall ; 

 the supero-marginal plates being also affected in the majority of cases. This feature at 

 once strikes the eye in comparison with the usually vertical and actinally well-rounded 

 margin of Ctenodiscus corniculatus and the thick and tumid one of Ctenodiscus australis. 



The paxillae of the abactinal area are small and crowded, similar to those in Cteno- 

 discus corniculatus. The madreporiform body is distinct and not hidden by paxillae as iu 

 Ctenodiscus australis. The marginal plates appear to be invariably rather more numerous 

 than in Ctenodiscus corniculatus, and consequently still more so than in Ctenodiscus 

 australis; — for example, in a specimen of Ctenodiscus procurator, measuring E, = 28'5 

 mm., there are eighteen supero-marginal plates counting from the median interradial line 

 to the extremity ; whereas in Ctenodiscus corniculatus of exactly the same radial dimen- 

 sions (R = 28'5 mm.) there are only fifteen. Ctenodiscus corniculatus, with R = 27 mm., 

 has fourteen supero-marginal plates; Ctenodiscus procurator, with R = 27 mm., has 

 seventeen. Ctenodiscus procurator appears to have generally one or more spines less on 

 the adambulacral plates than in Ctenodiscus corniculatus, three only being actually 

 marginal or furrow spines, and a fourth standing backward and on the actinal surface 

 of the plate at the aboral end. Very rarely indeed are four furrow spines present ; 

 whereas four and five are general in Ctenodiscus corniculatus. 



From the foregoing remarks it will be seen that Ctenodiscus procurator is much more 

 closely allied to the North-Atlantic Ctenodiscus corniculatus than to the comparatively 

 neighbouring form Ctenodiscus australis, from which it is readily distinguished. On the 



