418 PROCEEDIIs'GS OF THE NATIONAL MUSEUM vol.86 



rounded median projection: all four corners rounded; the plates are 

 more or less in contact at the sides but not in the median line. Side 

 arm plates low, not meeting distinctly either above or below; each 

 carries a series of 6 to 8 slender, blunt, somewhat flattened arm spines, 

 which are conspicuously thorny, at least along the sides; the upper- 

 most spine is very small, but sharp and thorny; the second is more 

 than twice as long, the third is much longer still, and either it or 

 the fourth is the longest of the series, the length exceeding the width 

 of the arm ; succeeding spines are shorter and more slender, the lowest 

 one or two (sometimes three) being notably slender, more or less 

 smooth except at tip, and blunt. Tentacle scale minute, flat and 

 rounded. 



Color of dr}' specimen pale gray on disk and upper arm plates, the 

 disk granules somewhat lighter: at base of arm the middle of each 

 upper arm plate is veiy slightly lighter than the sides and one can 

 almost detect a wide whitish longitudinal line thus marking the upper 

 surface of the arm. But this is to be detected only in the best light : 

 farther out on the arm each plate has a faint ill-defined whitish area 

 at center and on a few plates this has the shape of an hourglass; 

 there are no dark lines or markings on the upper surface of either 

 disk or arms. Arm spines glassy at base but becoming distinctly 

 pink at the tips. Oral surface nearly white, but the interbrachial 

 areas are gray and the under arm plates and the spines have more or 

 less of a pinkish tinge. 



Holotype. — Station .56: Barbados; Pelican Island, Carlisle Bay; 

 cracked from old blocks of coral: W. L. Schmitt, A])ril 19. 1937 

 (U.S.N.M. no. E. 5591). 



Remarks. — This curious little Ophiofhrh' was firet recorded by 

 Austin H. Clark (1921. Univ. Iowa Studies in Nat. Hist., vol. 9, no. 

 5, p. 54) under the name of O phiothrix Imsafa. In 1933 (Handbook 

 of Littoral Echinoderms of West Indies, p. 62) I made the futile sug- 

 gestion, based on the locality and habitat, that the specimen "must 

 be a peculiar individual" of O. suen.Honii. I was much farther from 

 the truth than my colleague, for there is no doubt that platyactis is 

 much nearer to lineata than it is to snemomi. It is readily distin- 

 guished from all its "West Indian congeners by the disk covering, the 

 upper arm plates, the arm spines, and the coloration. The shape of 

 the upper arm plates and spines and the absence of dark lines on the 

 upper side of the arms separate it sharply from lineata and even 

 more readilv from suensonii, which it does not resemble at all. 



U.SSOVERNMENT PRINTINS OPriCC: l*l* 



