330 VERRILL 



Porcellanasterinffi and some of the Astropectinidse, especially Lepty- 

 chaster. From the latter it differs in the somewhat more specialized 

 fascicles of the marginal and actinal plates, and their thinner struct- 

 ure and naked central portions. From the Porcellanasterinse the dif- 

 ferences seem important. The latter have much more specialized 

 fascicles, in special locations on the margin, with none between the 

 actinal plates, and usually lack regular dorsal paxillae. Like the 

 latter, it has single ambulacral ampullae. 



Genus Ctenodiscus Miiller and Troschel, 



Ctenodiscus Muller and Troschel, Syst Ast., p. 76, 1842. Sladen, op. cit., 

 p. 170, 1889. Fisher, op. cit., 191 16, p. 31. 



Stellate with nearly vertical margins. Actinal plates thin and flat, 

 without granules ; covered with membrane ; the principal ones united 

 in double radial rows, each double row corresponding to a single 

 marginal plate and to two adambulacral plates. Thus the fasciolate 

 grooves run between the double rows (or alternate single rows). 

 But distally, on the rays, the adambulacral and marginal plates cor- 

 respond, plate for plate. 



Dorsal ossicles are true paxillae. The upper marginal plates arc 

 high, thin, nearly vertical, and prcximally form a narrow margin, 

 usually with a small spine to each plate ; but distally they encroach 

 upon the upper side of the rays. 



The central dorsal area often has a more or less conical elevation 

 of the dermis, covered with paxillae like the adjacent parts, but finer. 

 It is more prominent in the young, and when the stomach is filled 

 with soft mud, as is usually the case when living. 



The northern and arctic species (C. crispatus) is circumpolar. 

 Two closely allied species or varieties are found in Antarctic waters, 

 one on each coast of southern South America. Dr. Fisher thinks 

 that they may not be distinct from C. crispatus. 



CTENODISCUS CRISPATUS (Retz.) Duben and Koren. 



Plate V, figures 6, 7, 8, 8a (details) ; plate xlix, figures 5, 50 (details). 



Aslerias crispatus Retzius, Dissert. Asteriarum, p. 17, 1805. 



Ctenodiscus polaris and C. pygmceus Muller and Troschel, Syst., pp. 76, 129, 



pi. V, fig. 5, 1842. 

 Ctenodiscus crispatus Duben and Koren, K. Vet. Akad. Handl., p. 253, 1844. 



Stimpson, Invert. G. Manan, p. 15, 1853. Lutken, Gronl. Ecliinod., p. 45, 



1857. Verrill, Proc. Bost. Soc. Nat. Hist., x, p. 345, 1866; Amer. 



Naturalist. XLni, p. 548, 549, figs, 3, o, b (4-rayed and s-rayed), 1909. 



