225 
III. Thenarocrinus callipygus . 
breadth at summit 16*57 millim. ; height of cup 8*71 millim. 
Specimen 293 Holcroft has — breadth at base 4*5 millim. ; 
breadth at summit 10 millim. ; height of cup 5*5 millim. : 
these proportions 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 flattening 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 And that the mean measurements and 
true proportions of the dorsal cup are: — Breadth at base 7*14 
millim.; breadth at summit 11*9 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 Holcroft — 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. 
Radials, 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 tegmen. 
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 *16 millim. wide. In 57478 a the free brachials 
