REPORT OF THE DIRECTOR I915 I27 



Other places in our figures. With apical plates in contact with floor 

 plates, there must needs have been passages for flow of coelomic 

 fluid, for nerves, etc. The study of other well- weathered material, 

 even fragmental, or of series of sections must be undertaken before 

 any decided opinion is expressed concerning the nature of these 

 apparently irregular passages. 



The arrangement of the flooring plates in U . medusa is 

 for the greater part an alternate one. Those oppositely placed being 

 those of the peristomial ring and a few following them ; arm A 

 has nine pairs oppositely placed, arm B but two pairs and arm C 

 apparently but one pair. In U . p u 1 c h e 1 1 a , however, in both 

 holotype and plesiotype, the arrangement so far as seen is an 

 opposite one. Only confusion can come from persisting in using 

 this character, of alternate or opposite arrangement of floor plates, 

 in our definitions of genera or larger groups of paleozoic sea stars. 

 In Blastoidocrinus, the blastids, and all forms in which the growing 

 arm tip rests against bibrachials, radials or terminal plates, the 

 flooring plates are developed alternately. When one has become 

 well grown and stiffened with stereom it takes up the thrusts 

 against the growing arm tip and leaves a space on the opposite 

 side, practically free from compression, where the embryonic new 

 plate and its concomitant structures may assume their proper 

 positions. The subsequent arrangement of these plates is due to 

 other factors and they may be found alternately placed in one 

 arm while oppositely placed in another of the same individual. 



Cover Plates 

 Schuchert (191 5, page 174) calls these plates "coin-shaped" 

 but the term is rather misleading. Plate i, figure i, lower left, 

 shows the proximal and outer surfaces of one of them. The form 

 here approximates that of a parallelopiped. The apical face rests 

 against an outer beveled portion of the oral surface of a floor 

 plate. The opposite or oral face is roughly semicircular and bears 

 a median crest. It is this part of the plate alone which suggests 

 the coin shape when the arm is in closed condition. The apical 

 faces of fifteen cover plates are shown in figure 2. These faces 

 are ridged or grooved transversely and show remains of muscle 

 fibers. The outer surface (as viewed in cross section) is either flat 

 or convex and the form taken seems to be due to the manner of 

 meeting the inframarginals. The last inframarginal on the right 

 shows one of these contacts which here gives a convex outer 

 surface to the cover piece. The portion of the inner surface next 



