46 THE VOYAGE OF H.M.S. CHALLENGER. 



much larger conical spinelet, and one or two or sometimes more thornlets. The miliary 

 spinulation of the infero-marginal plates consists of sharp conical thornlike granules widely 

 spaced. The mouth-plates have a marginal series of seven or eight spinelets, the inner- 

 most the longest, the rest very short and little more than flattened dog-tooth-shaped 

 spinelets ; there is also a lineal series of five or six small thornlets on the actinal surface of 

 the plate, parallel to the median suture. The tube-feet have a large, thick, and some- 

 what flattened terminal knob. 



Individual Variation. — This species is remarkably constant in its general characters, 

 so much so that out of a large series of specimens from one locality (Station 46) I have 

 scarcely noticed any features, even of trivial importance, that present differences worth 

 mentioning, excepting, of course, the normal modifications attendant on age and growth. 

 It may be remarked, however, after a careful examination of this material, that a slight 

 variation occurs in the length and delicacy of the central spinelet of the paxillse of the 

 area of the disk ; that occasionally one or two miliary spinelets below the large lateral 

 spine on the infero-marginal plates are more strongly developed in some examples than in 

 others : and that there is a slight variation in the length and robustness of the single large 

 spine on the actinal surface of the adambulacral plates (in one example they are nearly as 

 large as the spines on the marginal plates, and consequently very conspicuous). There is 

 also to be observed in some specimens a short and delicate miliary spinelet on one or even 

 sometimes on both sides of the actinal spine, but this is apparently of very rare occurrence, 

 and the companion spinelets are indeed so small that their presence can scarcely be said to 

 modify the "formula" of the adambulacral armature. 



Locational Variation. — In a small series of specimens from Station 50 the rays appear 

 to be proportionally rather narrower throughout when compared with examples of the 

 same size from Station 46, and the interbrachial arc is consequently more distinctly rounded. 

 The length of the major radius in proportion to that of the minor radius is also somewhat 

 greater in the specimens from Station 50. 



The few specimens from Stations 44 and 45 are of much larger habit than any of the 

 other's. The radius of the disk of the largest measures 15 mm., and the breadth of a ray 

 near the base (between the second and third supero-marginal plates) is 12'5 mm. ; unfor- 

 tunately the tips of the rays are broken away in all these specimens. There is a short but 

 robust conical secondary spinelet below the lateral spine on the infero-marginal plates on 

 the inner half of the ray, and sometimes even a second is present, the disposition being 

 then irregular. On the innermost two or three plates on each side of the median inter- 

 radial line the true lateral spines are reduced to the size of secondaries. The miliary 

 spinelets on the actinal surface of the adambulacral plates are usually more numerous 

 and spiniform, and the one that stands beside the large actinal spine is sometimes well- 

 developed. 



Localities. — Station 44. Off the coast of North America, east of Delaware and Maryland. 



