32 



BULLETIN 16, UNITED STATES NATIONAL MUSEUM. 



seven superimposed lamellae on either side. Marginal plates eleven to twenty, in 

 each series, from metlian interradial line to extremity of ray. A single short conical 

 spine at upper end of each superomarginal, and another similar one on each infero- 

 marginal on the actinolateral margin of ray. Adambulacral plates with an oblique 

 series of three to five sharp short skin-covered spines, and on aboral outer corner a 

 similar, usually shorter spine, covered with the general investment of actinal surface. 

 Mouth plates prominent: along free margin, about six spines like those of adambu- 

 lacral and at inner end of plate a single more prominent spine; two or three short 

 conical tubercles usually stand in a series on either side of median suture, these 

 sometimes as long as furrow spines. Superaml)ulacral plates present. 



Description. — No adequate description of this species is readily available. With 

 the ample material at my disposal it seems well to give a description and at the 

 same time to point out some of the most prominent of the variations. Instead of 

 placing these last, a separate subhead is given to each category of characters and the 

 variations considered at once. 



This is a remarkably variable species, especially in the length of the rays, and 

 their width, and in the general facies of the animal. Practically all of the most 

 diverse variations may occur in examples from a single station, and they are thus not 

 due to locality. As slender and broad armed forms occur among the smaller speci- 

 mens this difference is not due to age. 



Proportions. — A striking series of four nearly equal sized specimens from station 

 42.3.5 illustrates admirably the difference in form, measurements being given in the 

 accompanying table. 



Merisnrcmcnts of Ctenodiscus crispatus. 



16.0 

 14.0 

 17.0 

 19.0 

 18.5 

 17.6 



3.16:1 



2.45:1 

 2.57:1 

 2.35:1 

 1. 8:1 

 1.84:1 

 1.66:1 



Number 



ot supcr- 



omar- 



Einal 



plates." 



16-18 

 17-18 



Width 



of ray 



at base.' 



Number 



of 

 furrow 

 spines.o 



Actinal area very narrow; paxlllfe medium 



sized. 

 Paxillse medium sized. 



Do. 

 Paxlilae smaller. 



PaxilliE medium sized; average proportion. 

 Paxillae larger (I'l. 4, fig. 5). 

 Paxilla; small. 



» From interradial line to tip of ray. 



ft Measured from one interradial line, across abactinal surface to adjacent interradial line. 



c Spines on furrow edge of adambulacral plate; one or more spines on surface of plate not coimted. 



Specimen 423.5' has slender, narrow rays (from above resembling a Psilasfer 

 somewhat) and numerous marginal plates with fairly wide fasciolar furrows. The 

 intergradation is perfect, through 4235^ to 4286, a very short-rayed form almost 

 arcuately pentagonal, with few superomarginal plates. If, in the above table, the 

 width of ray at base is compared with R, and the porportion R :r taken into account, 

 the great difference in proportion is at once evident. 



Ahnctinal surface; paxillx. — In .some specimens the epiproctal cone is inverted, 

 in others very inconspicuous. In specimens with rays nearly the same length the 

 width of the paxillar area varies considerably, especially at the end of ray, thereby 



