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



Like a?, it supports three smaller plates, and from them spring 

 three longitudinal series of plates. 



The ventral sac is rarely exposed, but has been laid bare 

 by dissecting away part of the arms in E 1328 B. M. 

 (PL XIII. fig. 6). It is straight and its length in this speci- 

 men is 29 millira., i. e. about | length of arms. It is com- 

 posed of an unknown number of longitudinal rows of ossicles, 

 of which rows five can be traced down to the anal x. The 

 ossicles are primitively of hexagonal shape, but for the most 

 part are folded on the lateral margins ; in the median dorsal 

 region this lateral folding is as a rule slight, but in the other 

 rows it is strongly marked, and the appearance produced is 

 exactly that of Thenarocrinus^ but on a much smaller scale. 

 This minuteness renders it very hard to see that the folds are 

 not slits ; the appearance is most deceptive (PI. XIII. fig. 7). 

 The height of these folded plates is about *8 millim., their 

 width about 2 millim. Distally the sac tapers gradually to 

 a rounded point, the plates lose their folding and resume the 

 regular hexagonal outline. The exact position of the anal 

 aperture is not shown by any of the specimens examined ; 

 certainly no trace of it can be seen in the proximal region of 

 the sac. 



The Stem is longer and more slender than in other known 

 species of the genus. In E 1412, where it is completely 

 preserved, the measurements are : — Length 113 millim., 

 diameter about 2'5 millim.; the length of the crown is 39 

 millim. In the young specimen E 5611 (PI. XIII. fig. 5) 

 57 millira. are preserved, and the length of the crown is 23 

 millim. The usual diameter in mature specimens is 3 millim. 

 The section is round, or, in the proximal region, obscurely 

 pentagonal. In the proximal part of the stem the ossicles 

 alternate in diameter, and close to the crown the wider ossicles 

 are also slightly higher than the others ; lower down both are 

 of the same height ; in the distal region all the ossicles are of 

 equal diameter and equal height, and the height is rather less 

 than in the proximal region. The height of the ossicles in 

 the proximal half of the stem is usually a little less than '6 

 millim. In E 5611 (PI. XIII. fig. 5) the stem has been bent 

 sharply into three unequal portions; in the proximal portion 

 17 ossicles go to 5 millim., in the middle portion 13 ossicles, 

 in the distal portion 16. The sutures are finely crenelate, 

 indicating the presence of radiating stride on the articular 

 surfaces. The lumen is pentagonal, with angles radial in 

 position ; in the stem-section of E 5130, which is 2*5 millim. 

 in diameter, it has a diameter of "7 millim. (PI. XIII. fig. 11). 



There are five radial longitudinal sutures passing out from 



