PA Mr. F. A. Bather on British Fossil Crinoids : 
the arms of a single specimen; but, in Silurian forms at all 
events, each species has usually its own limits. 
Anal structures.—Anal & from tetragonal to hexagonal; it 
rests on the upper side of post.B, is in line with RR, and 
about 3 their width. In typical species it supports, by its 
humane upper side, a smaller plate of similar shape, while 
on either side of it, in the angle between it and the adjacent 
radial processes, rests a smaller plate of the tube (rt oa lt). 
Sometimes 7¢ and /é appear not to touch @ at all, in which 
case @ is four-sided. Sometimes (e. g. C. multibrachiatus 
from the Keokuk) the upper side of 2 is sloped downwards 
in such a manner that only ré touches the RR, in which case 
x is roughly five-sided. 
The Ventral Sac consists of more or less hexagonal plates, 
arranged in fairly regular longitudinal rows. It varies very 
ereatly in size, but appears never to extend to the length of 
the arms. It is rounded or swollen, and has a rather large 
lumen. The plates are solid, often slightly tumid, and some- 
times show a radiating structure, which may even exhibit 
itself in slight folding. In typical species of the genus the 
foldings are never pronounced, nor are the plates trans- 
versely elongate; in none are there slits or pores. 
The Tegmen comprises 4 Deltoids and a Madreporite 
(p. 211, fig. 6). The Deltoids rest on the radial processes, 
and abut laterally on one another and on the Madreporite. 
The Madreporite is usually cordiform and appears to be 
pierced by numerous pores. 
Ambulacrals (I Amb) pass between the deltoids and madre- 
porite to the actinal centre, in which region they are usually 
enlarged (and are by some writers considered to be the Orals). 
Smaller Interambulacrals (7 I Amb) are also often present, 
almost entirely covering the deltoids (p. 211, fig. 7). 
The Stem is rarely preserved to any extent, but it seems 
never to have attained a very great length. It varies much 
in width; it is round; with a usually quinquelobate lumen, 
sometimes of large size. Radial sutures have not been 
observed. 
Columnals rather low, and alternating in thickness and 
height ; or very low and equal in size. They have radiating 
striz on their articular surface. 
‘There are no Cirri on the stem. 
The Root has not yet come beneath my observation. 
SPECIES OF THE GENUS. 
Although the Austins in 1846 could deny the existence of 
