226 Mr. F. A. Bather on British Fossil Crinoids : 



are postpalmars of at least the sixtli series; in other words 

 the dichotomy is seen to take pLice at least 8 times, so that 

 the final brandies of the arms can have numbered no less than 

 1280, and were probably nearer 2000. The ossicles are of 

 peculiar shape ; even in the more proximal series their sides 

 are seen to curve round in a curious manner towards the 

 ventral surface, as is well shown by specimen 57478 h ; and, 

 as dichotomy progresses and the transverse axis of the ossicles 

 shortens, the dorso-vcntral axis becomes much longer, so that 

 in the first postpalmars the ratio of depth to width is as 5 to 

 3, and in the third postpalmars as 7 to 3 ; this is clearly seen 

 in specimen 144 Mason College (PI. X. fig. 1). In most 

 specimens the backs or outer portions of the arms present a 

 continuously smooth appearance, but in others the edges of 

 the ossicles are more rounded, inducing a slightly moniliform 

 aspect; the smoothness may therefore be due to attrition. Be 

 this as it may, there can be little doubt but that the arm- 

 ossicles are more ridged at the sides of the arms than on their 

 backs, and this in such a manner that the greatest transverse 

 diameter of each ossicle is towards its upper or distal end, and 

 towards its ventral surface. The condition of the specimens 

 does not permit the direct demonstration of a dorsal canal ; 

 but that such existed seems certain not only from the shape of 

 the ossicles, but also from the fact that in much weathered 

 specimens a groove is formed in the median line on the dorsal 

 surface ; this is best shown by specimens 138 Mason College 

 and 431 Holcroft (Diagram 8). The ventral surface of the 

 arms is partially exposed in specimen 57478 h : the covering- 

 plates are no longer in situ, and the food-groove, which is 

 rather shallow in proportion to the depth of the ossicles, is 

 clearly seen ; on either side of it the ventral edges of the 

 ossicles rise up like little rounded teeth (Diagram 9). 



Costals, 3 to each ray ; in two instances out of the twenty- 

 seven counted there appear to be 4 ; all of the same widtli, 

 which is always more than two thirds that of the radial ; height 

 from 1 to 2 millim. according to size of specimen. The upper 

 and lower edges of the second costal are straight and parallel ; 

 the lower edge of the first costal is curved conformably with 

 the articular facet of the radial ; the lines containing the 

 axillary angle of the third costal are concave. The upper 

 and lower edges of the costals are slightly bevelled on the 

 outside, indicating that these ossicles were united by loose 

 suture. 



Distichals vary in number in the different specimens, and, 

 to a loss extent, in different branches of the same specimen ; 

 19 branches have been observed with 4 distichals apiece, 11 



