8 
MUSEUM BULLETIN No. 31. 
Chapman’s original specimen, collected at Peterborough, was o£ 
the straight-rayed variety. The species though fairly common has 
never been properly described or figured. Chapman’s name is here 
restricted to the form with straight rays and the type of ornamenta- 
tion 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 
enclosing the anal inter-radius a little farther apart than the others. 
Each 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 alternating pores are seen between 
the covering pieces. Over the central area, presumably covering the 
mouth, 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 Tays 
numbered in clock-like (solar) order, beginning with the one at the 
left of the anal inter-radius. 
i 
The single large plate of the supra-oral series is then between 
rays I and Y, and its great width is due to the enlargement 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 TV and Y. These at their proximal 
edges abut against the anterior supra-oral plates. Numbering them 
according to the inter-radial areas which they oppose, the broad 
posterior one is 5, the next one to the left 1, the first anterior lateral 
2, second anterior lateral 3, and the right posterior 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 lattei 
appear to be five in number, and to constitute the first ray plates, 
one being common to two adjacent rays.” 
No two of the covering-plates, either of the rays or of the supra- 
oral system, seem to be in any way joined together, but all were 
probably 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. 
