264 PROCEEDINGS OF THE ACADEMY OP [1881. 



length ; in others thickened and rounded above, and slightly 

 thinner below ; in some terminating in thin round edges, in others 

 tapering almost to a point ; while still others are bifurcate at the 

 extremities. The monstrous plates, which rise to the full height 

 of the summit, and laterally extending far beyond it, rest chiefly 

 upon the surface of the vault, within grooves, bordered by elevated 

 ridges along the interradial and proximal dome plates, and only a 

 small portion at the lower end is wedged in between other plates. 

 The ridges continue along the interbrachial plate, which for a 

 plate of that order is unusually large. The interbrachial plates, 

 together with the secondary radials and lower interradials, form 

 the base of the dome. 



The summit leans somewhat to the posterior side, more espe- 

 cially the upper portion, which in the best specimens consists of a 

 small cone, composed of a number of small plates, which decrease 

 in size ui)ward, leaving a minute anal opening at the upper end 

 of the. cone.' 



In front of the anal opening, and in the radial centre, there is a 

 pentagonal plate which is at once recognized as the central dome 

 plate. To this plate converge not only the radial grooves in 

 which the winged processes rest, but also the smaller grooves 

 within the interradial spaces which receive the arms. These 

 latter pass into the ambulacral or arm openings, which are rather 

 large, and have an upward direction. 



The arms are short, simple, gradually diminishing in size up- 

 ward, extending to the top of the vault, but not beyond it. They 

 are twenty in number, divided by the winged processes into groups 

 of four, each containing two arms of two different rays. They 

 are constructed of two rows of short interlocking joints, moder- 

 ately convex on the dorsal side. Ambulacral furrows, wide and 

 deep. Pinnules short, stout, composed of five or six joints. 



The visceral cavity, as seen from one of Wetherby's specimens, 

 (vertical section) is deeper than would be expected from the form 

 of the body. The basal plates are very thin, while the radials, to 

 the top of the third order, increase rapidl3^ in thickness. 



Column slender, round ; central perforation small. 



' It is very possible that in some of the species the anus is not thus ex- 

 tended into a tube-like cone, but this is the case in Pterotocrinus depressus 

 Lyon and Cass. The anal aperture is but rarely observed, being generally 

 covered by the shell of a Gasteropod. 



