186 THE CANADIAN NATURALIST. [June 



columnar pit; always concealed when the column is present. 

 These are surrounded by three other plates, the six altogether 

 corresponding to the six pieces which constitute the compound 

 basal plates of Pentremites. They are represented at fig. 6, h, as 

 figured by Mr. Lyon (Geol. Ky., vol. iii, pi. v, fig 1, h.) 



In the next series there are five plates which are undoubtedly 

 the homologues of the five forked plates of Pentremites. They 

 are very short and confined to the base of the body. They form 

 a shallow basin with ten re-entering angles in its margin. Fig. 6,/. 



Alternating above the forked plates, are five pieces correspond- 

 in"- to the deltoid or interradial plates of Pentremites. Some of 

 these are lanceolate in form (fig. 6, d), their broader extremities 

 fitting into the angles between the forked plates. They taper to 

 a point upward, and their sides are bevelled so as to pass under 

 the ambulacral plates, to which they are, in general, so closely 

 united, that the line of junction is indicated only by the difference 

 in the markings of the surface. Owing to this structure, these 

 plates have not always been recognised by the authors who have 

 described this genus. They were first pointed out by Mr. Lyon. 

 The fifth deltoid or interradial plate is truncated at its apex for 

 the reception of the oro-anal orifice (??iv, figs. 4, 6). The sutures 

 on each side of this plate are generally distinctly visible, especially 

 in the upper part of the body. 



The ambulacra are narrow — one line wide in a specimen fifteen 

 lines in length, with a fine median groove, about large enough to 

 accommodate a tube of the size of a horse-hair. There are two 

 rows of pores, those on one side of the groove alternating in posi- 

 tion with those en the other side. These pores lead into the 

 hydrospires. There appear to be only two rows of ambulacral 

 ossicles. The pores are situated in the sutures between them. 

 On each side of the ambulacrum there is a broad transversely 

 grooved marginal plate. From each pore a small rounded ridge 

 runs across this plate. The grooves between the ridges originate 

 at the outer extremities of the ambulacral ossicles. In well-pre. 

 served specimens the surface of these marginal plates exhibits no 

 other structure than the transverse grooves and ridges; but in 

 one weathered specimen that I have examined, they seem to be 

 composed of a number of narrow elongated pieces, arranged trans- 

 versely in such a manner that two of them abut against the outer 

 extremity of each of the ambulacral ossicles, and extend outward 

 toward the interradials. This seems to prove that the marginal 



