IV. Thenarocrinns gracilis. 37 



wide. Arras about six times length of cup, slender, dicho- 

 tomize about five times ; arm-ossicles, when seen from back, 

 about as high as wide ; lateral compression very slight. 

 Anal plates situated low in the cup. Stem (so far as known) 

 smooth. 



The graceful and slender appearance suggests gracilis as a 

 natural nomen triviale. 



Description of Specimen. 



Dorsal cup forms a cone the sides of which are almost in a 

 line with the gradually tapering proximal part of the stem 

 below and witli the arms above ; thus its elongate character 

 is exaggerated. The measurements are : — Breadth at base 

 3'2 millira. ; breadth at summit 6*5 millim. ; height of cup 

 5'5 millim. The specimen, however, is obviously flattened 

 in the plane of bedding ; the stem, for instance, is so crushed 

 that its section, instead of forming a circle, is an ellipse with 

 axes in the proportion of nearly 3:2; the cup, as one 

 would expect, is crushed more than the stem, and the 

 proportions may be taken as quite 3 : 2. 'J'he true measure- 

 ments would thus be : — Breadth at base 2"7 millim. ; at 

 summit 5*4 millira. Consequently the angle which the side 

 of the cup makes with the long axis is about 13° 47'. 



IB, presumably 5, of which 4 should be pentagonal. Only 

 the two posterior are seen : r. post. IB. pentagonal, with 

 height about equal to breadth; 1. post. IB. abnormally wide, 

 of somewhat uncertain relations, but probably with truncate 

 top supporting post. B. 



B. presumably 5 and hexagonal. Only post. B. is com- 

 pletely seen ; portions of r. and 1. post. BB. are visible. 

 They would appear to have been about as high as wide. 

 The post. B., however, is higher than wide, and its distal 

 angle is truncated by a small plate which seems to belong to 

 the tegmen : this is probably an individual abnormality. 



R. presumably 5, though only 2 are seen : higher in pro- 

 portion than in type species, with articular facet apparently 

 more excavated. The I. post. R. is cut short on the side next 

 the anal area by the tegminal plate that was mentioned as 

 resting on the post. B. : this feature also is probably an 

 individual abnormality. 



Arms have, by reason of their slenderness, a longer appear- 

 ance than those of T. callipygus ; but, owing to the greater 

 height of the cup, their length relatively to it is less, namely 

 six instead of seven times its height. The extent of their 

 dichotomy cannot be exactly ascertained, but their free 



