MONOGRAPH OF JAPANESE OPHIUÜOIDEA. 149 



within, outer side decidedly convex, outer angles rounded ; succes- 

 sive plates separated by the lateral arm plates, except the basal 

 two or tlii-ee. Lateral arm plates low, not very prominent. First 

 ventral arm plate small, divided into two pieces, of which the 

 inner one is triangular and the other quadrangular. Those beyond 

 large and liexagonal, except the second plate which is pentagonal ; 

 much wider than long, widest at the outer lateral angles, with 

 concave lateral sides and slightly convex inner and outer sides, 

 swollen along the outer borders and especially at the outer angle, 

 so that the arms appear keeled along the ventral median line. 

 Three arm spines, subequal, about as long as the corresponding 

 arm joint, but the uppermost one is slightly longer ; cylindrical, 

 tapered and blunt. Two flat, oval tentacle scales to each pore, 

 but sometimes tliree to the first. 



Colour in alcoliol white. 



The present species may be easily distinguished from all the 

 other species of Amphiactis by the Ojjhiozoiia -like squamation, and 

 especially by the presence of the umbonated primary plates. 



Family 2. Amphiuridae (Ljungman, 1867) mihi, 1915. 



Disk covered with fine, imbricating scales, or rarely by a 

 naked skin, sometimes beset with minute spines. Eadial shields 

 well developed, with a conspicuous articular socket on the ventral 

 surface near the outer end, fitting to the large, ball-like articular 

 condyle of the genital plate. Genital plates firmly fixed to the 

 basal vertebrae. Genital scales short, wide, flat, articulating with 

 the genital plates near the outer end of the latter. Besides, there 

 occur a pair of short, flat scales, just outside each oral shield, 

 supporting the proximal abradial border of the genital shts. 

 Peristomial plates very small, usually entire. Oral frames very 



