8S CANADIAN FOSSILS. 



edge, Sd, is furnished with one, two, or more transverse plates, which 

 also appear granular, supporting other smaller ones which appear 

 to fill up all the space over the excavations, and to form a sort of 

 polygonally plated tube. This tube is nearly complete at a, fig. 1, 

 and from near this point a similar but truly cylindrical tube extends 

 out beyond the ring of ossicles, and is seen at b complete, and at c 

 with its upper surface broken away, forming a continuous tube fully 

 three-quarters of an inch beyond the disk, and apparently not termi- 

 nated even then ; 2d* shews it magnified.t 



3. Fig. 5 shews a still more complete development of this mar- 

 ginal plating, and the difference in size of the basal plates (see fig. 5* 

 and the marginal ones, b) is such as to suggest the idea of a distinct 

 species. It is perhaps only a younger specimen, considerably abra- 

 ded, but may however receive provisionally the name of C. depressus. 

 The specimen 5 shews a considerable portion of the loosely- 

 arranged and flattened ossicles of the disk, which are not closely 

 plated, but leave large interstices between them. This structure is 

 better seen in the small fragment of the central disk of figure 2 

 (magnified in figure 4), in which the granular surface of the plates, 

 and their irregular arrangement with interstices, is very distinct. 



Figs. 8 and 9 shew still more clearly the radiated stincture of 

 the upper surface in another species. 



Fig. G is the opposite or under-side (at least the flatter and less 

 ornamented side), and may be presumed to be C. Halli. The great 

 ossicles here are marginal, and have no plated integument stretching 

 out beyond them, the outer edge, 66, corresponding to the edge, M, 

 on the opposite face. 



The silicified specimen, 6, has been macerated in muriatic acid to 

 expose it, and has suffered abrasion — neither the marginal ossicles, 

 6, nor the reticular surface, a, shewing so clearly as they should do 

 the real structure of the surface. At a a portion of the upper-side 

 is seen, viz., impressions of three of the large granular ossicles, 

 with their external deep pits (represented in the cast as circular 

 elevations, with a minute central pit in each). 



It is this specimen which so satisfactorily explains the meaning of 

 the cast next to be described, and which, though difiering specifically 

 and from another formation, also helps materially to elucidate the 

 specimens already described. 



t Upon a re-examiuation of this spBcimeii I can detect no connection between the 

 fragment of the tube and the disc, althousrh it mav have been connected. — E. B. 



