212 Mr. Y. 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. 



yinnl structures. — Anal x from tetragonal to hexagonal; it 

 rests on the upper side of post.B, is in line with RR, and 

 about I their width. In typical species it supports, by its 

 horizontal upper side, a smaller plate of similar shape, while 

 on cither side of it, in the angle between it and the adjacent 

 radial processes, rests a smaller plate of the tube {rt and It). 

 Sometimes rt and It appear not to touch x at all, in which 

 case X is four-sided. Sometimes (e. g. C. muUihrachiaius 

 from the Keokuk) the upper side of x is sloj)ed downwards 

 in such a manner that only 7't 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 

 greatly 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. 



Hie Tegmen comprises 4 Deltoids and a j\Iadreporite 

 (p. 211, fig. C). Tiie 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- 

 ])orite to the actinal centre, in which region they are usually 

 enlarged (and are by some writers considered to be the Orals). 



Smaller Intcrambulacrals {i I Amh) 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 tliickness and 

 height ; or very low and equal in size. They have radiating 

 striai on their articular surface. 



There are no Cirri on the stem. 



The Root has not yet come beneath my observation. 



SrECIES OF THE GeNUS. 



Although the Austins in 1846 could deny the existence of 



