176 NEW YORK STATE MUSEUM 



branch not far witliin tlieir extinction, rather than the inter- 

 vening unporous areas : an appearance well depicted in several of 

 our figures but somewhat illusory as explained by the diagram 

 (plate 5). We have then, evidently, in these porous radial bands, 

 well defined ambulacral areas. The limits of their component 

 plates are almost completely obsolete, but notwithstanding the 

 general obscurity there are places where their margins are clearly 

 discernible. The character of these ambulacral areas of the first 

 cycle does not differ from that of later cycles, but their aspect 

 is different : the close crowding of the alternating knots or short, 

 horizontal ridges produces the braid-like appearance already de- 

 scribed, and the pores are only obscurely shown, perhaps because 

 of imperfect retention and perhaps from incomplete develop- 

 ment; but when visible they are seen to lie in the grooves be- 

 tween the horizontal knots. 



At the point of convergence of these radial bands near the 

 center of the disk we have looked in vain for any evidence of 

 structure. In three or four specimens this area is retained and 

 in these it is simply a smooth, structureless spot where with the 

 most obscure beginnings the ambulacral radii come into being. 

 It is difficult to, believe that the space did not possess some dif- 

 ferential structure, but in that event it was of so delicate detail 

 that it has been lost in the rather rude retention of these fossils. 

 Nor is there any other single spot or area on this dorsal disk to 

 which any special structure may be ascribed. Let us farther 

 emphasize this significant fact; outside of the ambulacral areas 

 there is no x)alpable evidence, either on the. dorsal or ventral 

 faces of the disk, of geometric plates, nor anywhere of a tubercled^ 

 scrobicular surface. 



All the evidence, then, that can now be educed from this fossil 

 leads to the following inference. The organism is probably an 

 echinoid. In form it is a flattened, oval disk, not unlike 

 S c u t e 1 1 a in this respect. In an uncompressed condition the 

 imperforate side may have been slightly concave; the other 

 surface was distinctly convex. Its ambulacral region is re- 

 stricted to one side, as in many clypeastroids and spatangoids. 

 The imperforate side bears radiating and inosculating surface 

 channels not unlike those of S c u t e 1 1 a. The ambulacra are in 



