JAPANESE ASTEKOIDEA. 103 



lateral margins are also covered with a similar membrane and 

 the two series of marginal plates can be distinguished from each 

 other only with difficulty. The individual plates of each series 

 are distinctly separated from one another by deep grooves. 



Superomarginals. — The superomarginal plates are covered with 

 a tumid membrane continuous with that of the actinal side, and 

 in surface view their boundaries towards the inferomarginal plates 

 are either invisible or indistinct. The individual plates of the 

 series are however very distinct and are separated from one 

 another by fasciolar grooves, which are in reality very spacious 

 but are largely covered over by the series of delicate spinelets 

 standing out at right angles from the keel of the plate and 

 connected together by a fold of the superficial membrane. The 

 outer face of the superomarginals is perfectly vertical or very 

 slightly inclined towards the actinal surface. In the interbrachial 

 angles the superomarginals are nearly one and a half times as 

 liigh as they are wide, but near the tip of the arms the width 

 and height are about equal. The lateral spinelets on either side 

 of the keel are 10-12 for each plate in the interradii (PL VII, fig. 

 115). At the abactinal end cf each superomarginal plate there is 

 a pointed, somewhat flattened, conical spine, which becomes 

 smaller towards the tip of the arms and is covered over by the 

 same membrane as the plate itself. On the lateral face of the 

 keel, i.e. in the fasciolar grooves behind the row of delicate spine- 

 lets above mentioned, there are three or four skin folds parallel 

 to the outer face of the superomarginal plate and to the row of 

 lateral spinelets, and they contain numerous flattened calcareous 

 rods of microscopic size (PL VII, fig. 118; PL VIII, fig. 125). 

 There appear to be some differences in the size and form of these 



