III. Thenarocrinus callipygus. 225 



hrcadtli at suniniit IG'57 mlUim. ; height of cup 8'71 millim. 

 Specimen 293 llolcroft lias — breadth at base 4'5 millim. ; 

 breadth at summit 10 millim. ; heiglit of cup 6'5 millim. : 

 tlicsc ]n-oporlions are practically the same as those of the 

 mature individuals. But since all these specimens are more 

 or less flattened the proportions are not those of the cup in 

 life ; nevertheless they will be found characteristic of most of 

 the fossils, and it must be remembered that this flatteninsj is 

 itself largely due to the structure of the cup. The proportion 

 of the cup in life may be gathered from the uncrushed speci- 

 men 153 Mason (College, of which the mean measurements 

 are : — Breadth at base 9 millim. ; breadth at summit 15 

 millim.; height of cup 11 millim. This cup is larger than 

 the others, but, taking its proportions as correct and re- 

 ducing them, we find that the mean measurements and 

 true proportions of the dorsal cup are:— Breadth at base 7'14 

 millim. J breadth at summit ll'O millim. ; height of cup 8'73 

 millim. Consequently the angle which the side of the cup 

 makes with the long axis is about 9?°. 



Infrabasals, 5 ; pentagonal, except r. post. IB., which is 

 hexagonal owing to truncation of distal angle ; mean measure- 

 ments — greatest width 5 millim., height 3 millim. Measure- 

 ments in 293 llolcroft — width 3'25 millim., height 1*8 

 millim. 



Basals, 5 ; hexagonal ; mean measurements, 6 millim. wide 

 by 5 millim. high ; width of 1. post. B. about 1 millim. less, 

 while r. post. B. is a little distorted. In 293 Holcroft the 

 normal basals are about 4 millim. wide by 3 millim. high. 



Kadials, 5 ; in general outline pentagonal, or more accu- 

 rately a hexagon of which the distal angle is truncated by a 

 wide reentrant curve for the articulation of the first costal ; 

 mean measurements — width 7"3 millim., height, from proximal 

 angle to middle of articular curve, 3'5 millim. In 293 Hol- 

 croft a fracture crosses the radial circlet, but the measure- 

 ments appear to be about 4 millim. wide by 2"5 millim. high. 

 The curve varies in width, sometimes occupying almost the 

 whole width of the radial, but never quite so little as two 

 thirds of its width. On either side of the facet the distal 

 portions of the radial bend inwards to meet the tegraen. 



Arms, to judge from specimens 4809 and 57478 a, about 

 seven times as long as height of cup : seen from outer or 

 dorsal surface, appear broad and flat-backed in the proximal 

 regions ; but undergo rapid dichotomy, and, as the ossicles of 

 each series are about five-sevenths the width of those in the 

 preceding series, in the distal regions are remarkably atten- 

 uate, being '10 millim. wide. In 57478 a the free brachials 



