66 



BULLETIN 75, UNITED STATES NATIONAL MUSEUM. 



which is marginal, can be distinguished with more or less ease. 

 Radial shields about as wide as long, more or less united distally; in 

 the type-specimen, several rounded knobs or papillae are borne on 

 the inner distal corners of the plates (some times also on the distal 

 margin) and in fig. 17 a these appear like three small plates separat- 

 ing the radial shields distally. Basal upper arm plates tetragonal, 

 but rapidly becoming triangular as they pass outward; in the type, 

 the first seven to nine are in contact but in others (smaller speci- 

 mens) only the first two or three. Interbrachial spaces below covered 

 by twelve to twenty plates of which the marginal are largest. Oral 

 shields pentagonal with rounded corners, about as wide as long. 



Adoral plates long and nar- 

 row ; oral plates considerably 

 broader than adoral but not 

 so long. Oral papillae about 

 five or six on a side ; those at 

 apex of jaw are pointed, but 

 outer ones are truncate and 

 may be wide and squarish. 

 Genital slits long and genital 

 scales wide; each scale has 

 a marginal series of small, 

 close-set, truncate papillae, 

 which are more elongated on 

 upper side of arm, where the 

 two series (of opposite sides 

 of arm) meet and form a 

 single unbroken series across 

 base of arm. First under 

 c arm plate rounded diamond 



FIG. 17. OPHIURA MONOSTCECHA. X5. a, FROM ABOVE; . 



b, FROM BELOW; c, SIDE VIEW OF THREE ARM JOINTS Sliaped; Second plate 



NEAR DISK. 



squarish; succeeding plates 

 pentagonal, becoming triangular; all these plates are wider than long, 

 and except first two are nearly or quite separate from each other. 

 Side arm plates large, meeting above and below, except at base of 

 arm; each plate carries three subequal, well-spaced, peg-like arm 

 spines, less than half as long as joint. Oral tentacle pores may or 

 may not open into mouth slit; in the type they do; they are pro- 

 tected by five or six scales on each side. There seem to be no 

 tentacle pores on arm distal to first three or four joints, and these 

 are guarded by three and two scales or by two and one; but through- 

 out whole length of arm, the lowest arm spine is accompanied by 

 a minute peg-like tentacle scale crowded close against it. Color 

 (dried from alcohol), pale gray or whitish. 



