134 



BULLETIN 75, UNITED STATES NATIONAL MUSEUM. 



These specimens have been compared with a cotype of Lyman's 

 and there is no doubt of their identity. The disk diameter ranges 

 from 11 to 22 mm. There are usually three arm spines present, 

 sometimes only two, but in the larger specimens there are often four. 

 The greatest diversity is shown in the shape of the mouth shields, for 

 while it is true that they are usually much wider than long, in some 

 specimens they are nearly oval in outline and quite as long as broad. 

 Intergradations between the two extremes are common. Specimens 

 with long oral shields have the adoral plates very indistinct or almost 

 wanting. The degree of carination on the under side of the base of the 

 arm is also subject to considerable variation. 



OPHIACTIS PTEROPOMA, new species.-* 



Disk 5 mm. in diameter; arms about 20 to 25 mm. long. Disk cov- 

 ered by rather coarse scales, 150 to 200 altogether, without any disk 



FIG. 50. OPITIACTIS PTEROPOMA. xio. 



a, FROM ABOVE; 6, FROM BELOW; c, SIDE VIEW OF THREE ARM 



JOINTS NEAR DISK. 



spines. Radial shields long and narrow, separated by a linear series 

 of three scales, or in contact at outer end. Arms six, broad at base, 

 but becoming attenuate at tip. Upper arm plates broadly triangular, 

 with rounded angles, much wider than long, not actually in contact 

 even at base of arm, though side arm plates do not meet between 

 them there. Interbrachial spaces below closely covered with scales. 

 Oral shields variable, broad, triangular, or top-shaped, with lateral 

 angles rounded. Adoral plates long and narrow, wider without than 



a Urspov, signifying wing, and retina, signifying lid, in reference to the wing-shaped 

 oral papillae. 



