100 BULLETIN 75, UNITED STATES NATIONAL MUSEUM. 



curved, triangular, twice as long as wide, with distal end rounded, 

 separated proximally by a large group of scales and distally by basal 

 upper arm plates. First three upper arm plates very short, much 

 wider than long, crowded between distal ends of radial shields ; fourth 

 plate longer and narrower, with a very convex distal margin ; succeed- 

 ing plates narrow, oval, very much longer than wide, only basal ones 

 or none in contact. Interbrachial spaces below, covered by oral 

 shields and numerous, small, imbricating scales. Oral shields large, 

 broad, pentagonal, about as wide as long, with lateral margins more 

 or less concave. Adoral plates long and narrow, wider within than 



FIG. 37. OPHIOCTEN OSPLAX. X5. a, FROM ABOVE; 6, FROM BELOW; c, SIDE VIEW OF THREE ARM 



JOINTS NEAR DISK. 



without; oral plates large, nearly triangular, markedly swollen at tip. 

 Oral papillae three or four on each side, narrow and pointed. Genital 

 slits long and conspicuous; genital scales narrow; in young speci- 

 mens each scale carries a marginal series of very minute papillae, 

 which become long enough on upper end of scale, visible from above, 

 to form a more or less distinct arm comb ; in adults this comb is either 

 insignificent or entirely wanting, and the genital scales are scarcely 

 visible from above. First under arm plate more or less hexagonal, 

 wider than long ; second plate more pentagonal, with a sharp proximal 

 angle and outer corners rounded ; succeeding plates more tetragonal 

 or triangular, very much wider than long; only first two or three 

 plates in contact. Side arm plates large, meeting above and below, 



