the Crinoidea, Cystidea, and Blastoidea. 147 



them cannot be distinguished. According to Mr. Lyon, who, 

 through his long-continued geological researches has collected 

 and studied a vast number of specimens, there are three mi- 

 nute lozenge-shaped or quadrilateral basal plates, situated at 

 the bottom of the columnal pit, always concealed when the 

 column is present. Tliese are surrounded by three other 

 plates, the six together corresponding to the six pieces which 

 constitute the compound basal plates of Pentremites. They 

 are represented at fig. 6 b^ as figured by Mr. Lyon (Geol. Ky. 

 vol. iii. pi. 5. fig. 1 h). 



In the next series there are five plates, which are undoubtedly 

 the homologues of the five forked plates oi 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 

 (%-6/). . 



Alternating above the forked plates are five pieces corre- 

 sponding to the deltoid or interradial plates of Pentremites. 

 Some of these are lanceolate in form (fig. 6 J), 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 ambulacra! 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 recognized 

 by the authors who have described this genus. They were 

 first pointed out by Mr. Lyon. The fifth deltoid or inter- 

 radial plate is truncated at its apex for the reception of the 

 oro-anal orifice (w<f, 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 position with those on the other side. These 

 pores lead into the hycU-ospires. There appear to be only two 

 rows of ambulacral ossicles. The pores are situated in the 

 sutiu'cs 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-preserved 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 naiTOw elongated pieces arranged transversely in such a 



