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and a row of them can sometimes be distinguished next to the 

 ambulacral series. The total number of the spines on the actinal 

 surface of each plate is some ten or twelve for the proximal two- 

 thirds of the arms. 



MoulJi- plates. — In denuded specimens the mouth-plates project 

 veiy prominently on the ventral surface. Each pair encloses a 

 a capacious space between covered with a membrane. Each plate 

 is long and bears on the actinal surface a conspicuous principal 

 ridge extending along the whole length of the plate and two short 

 accessory ridges near the mouth end, of which one is nearly 

 parallel to the main ridge while the other makes an angle of 

 nearly 60^-^ with it. In both of my specimens the mouth-plates 

 are much abraded iu the distal part ; but the arrangement of the 

 spines on the main crest appears to be as follows (PI. II, fig. 29). 

 There are three irregular rows, of which two extend along the whole 

 length of the plate, while the third extends only through the 

 distal half of the plate and occupies the outermost margin of the 

 crest. The first mentioned two rows are terminated at the mouth 

 end by a common long, robust spine, which with its feUow forms 

 a conspicuous pair in each interradius. The total number of 

 spines on the main crest appears to be some 40-45. Of the two 

 accessory crests, the one next the main crest extends through a 

 little over one-third of the length of the mouth-plate and bears 

 some 5 or 6 spines. The third crest which extends nearly vertically 

 towards the mouth is nearly as long as the other secondary crest 

 and bears about eight spines arranged in two rows. The three 

 crests meet at the base of the conspicuous mouth spine, and the 

 last mentioned crest can be seen clearly only in a lateral view 

 of the mouth-plate (PI. II, fig. 30). 



