1915] ■ The Ottawa Naturalist. 57 



now restricting Chapman's name to the form with straight rays 

 and the plate ornamentation described below. 



Description. 



Specimens small, circular in outline, not ordinarily resting 

 upon any foreign object. Rays five in number, narrow, straight, 

 and tapering but little toward the distal end, the two rays enclos- 

 ing the anal inter-radius a little further apart than the others. 

 Each ray has about thirteen pairs of alternately placed lateral 

 covering plates, which are truncated at the ends, so that they 

 interlock along the median line. The points of these plates 

 are curved, so that when the ray is slightly sagged apart, alter- 

 nating pores are seen between the covering plates. Over the 

 central area, presumably covering the mouth, there are three 

 principal plates, a large one next to the anal inter-radius and two 

 smaller ones anterior to it. 



For convenience in speaking of these fossils, the anal inter- 

 radius is called posterior, the ray opposite to it anterior, and 

 the rays numbered in clock-wise (solar) order, beginning with 

 the one at the left of the anal inter-radius. 



The single large plate of the supra-oral series is then, 

 between rays I and V, and its great width is due to the enlarge- 

 ment of the posterior inter-radius by the anal opening. The 

 other two plates are inter-radial in position, one being between 

 rays II and III, and the other between III and IV. There are 

 also two other narrow, five- sided plates accessory to the supra- 

 oral system, one between rays I and II, and the other between 

 IV and V. These plates at their proximal edges abut against 

 the anterior supra-oral plates. Numbering these plates accord- 

 ing to the inter-radial areas which they oppose, we have the 

 broad posterior one as 5, the next one to the left 1, the first 

 anterior lateral 2, second anterior lateral 3, and the right pos- 

 terior lateral 4. 



There can be no reasonable doubt that Chapman's specimen 

 had this structure. In his principal description, in the Ann. 

 Mag. Nat. Hist, he says: "These rays, at their origin, leave a 

 small central space covered by larger and somewhat rhombic 

 plates. The latter appear to be five in number, and to con- 

 stitute the first ray plates, one being common to two adjacent 

 rays." 



None of the covering plates, either of the rays or of the 

 supra-oral system, seem to be in any way joined together, but 

 were probably all movable. The three principal supra-oral 

 plates, Nos. 2,3, and 5, are of such form and strength as to 

 suggest that they could have functioned as jaws. 



The inter-radial spaces are covered with small imbricating 



