284 AET. 2. — H. MATSUMOTO : 



ce — Dorsal arm plates proper divided into halves, which are widely 

 separated from each other by a mosaic of supplementary plates ; 

 disk covered with very fine scales ; radial shields very small ; three 

 or four tentacle scales Ophioplocus. 



Key to Japanese species of OiJhioniusiuni, 



A — Two pairs of tentacle pores to each radius ; genital slits free of 

 papillse ; genital scales extremely stout ; a very conspicuous, large, 

 central plate is present in each interbrachial ventral surface ; disk 

 covered with very stout plates and very large radial shields. 



a — Genital slits very short and j)oi'e-like, or invisible, free from the 

 oral and adorai shields, which are completely joined ; no ventral 

 arm plates beyond the third. 

 b — Disk margin free of tubercles ; disk with some secondary plates, 

 besides the primaries and radial shields ; two or three arm spines. 



c — Disk plates more or less imbricated, or arranged like steps 



scalaire. 



ce — Disk plates tessellated simplex. 



hh — Disk margin beset with tubercles ; disk with only tlie primaries, 

 besides the radial shields ; five arm spines granosum. 



aa — Genital slits long, peculiarly situated between tlie oral and adorai 

 shields, the former being almost axe-shaped. 

 d — No ventral arm plates beyond the third. 



e — Disk covered with rather small primaries and smaller secondary 

 plates, besides the radial shields, which are separated from one 

 another ; dorsal side of the disk and arms usually very tubercular ; 

 interbrachial ventral surfaces covered with a large central and 

 several secondary plates, besides the stout genital scales. . . .lymani. 



ee — Disk with only very stout primaries, besides the radial shields, 

 which are joined ia pairs ; dorsal side of the disk and arms 

 smooth ; inter brachial ventral surfaces covered only by a very large 

 central plate, besides the very stout genital scales lunare. 



