IV. Thenarocrinus gracilis. 39 



to 1*66 millim. There is a very slight trace of a radial suture 

 in what would normally be the 1. post, radius, but, owing to 

 the abnormalities of the infrabasal circlet, this suture appears 

 to lie in the posterior interradius. The axial canal appears 

 to have been large, but its exact size cannot be determined. 



General Remarks. 



To our knowledge of the morphology of Thenarocrinus the 

 present species adds but little; the following points may, 

 however, be noticed. First, it entails a slight modification of 

 the diagnosis (I. c. p. 224), which must now read " R' in Basal 

 circlet, resting on one or more post. IB." Secondly, from 

 the evidence of this specimen, combined with that of speci- 

 mens of T. cattipygus, it is certain that at least four out of the 

 eight ridges of the ventral sac arise by dichotomy from the 

 brachianal. Lastly, we must mark the absence of a second 

 brachianal, for in nearly all genera of Fistulata that plate is 

 present : in the few cases where it cannot be distinguished, 

 in other words where the first brachianal is itself axillary, the 

 two plates may possibly have fused ; certain facts render 

 this explanation probable, but there is nothing in the present 

 specimen that makes either for or against it. 



The small size of this specimen, as compared with the 

 specimens of T. callipygus, may suggest that it died young. 

 But the fully developed plication of the plates in the ventral 

 sac is more powerful evidence of its maturity. 



Certain features in the present specimen, since they appear 

 to be abnormalities rather than specific characters, find no 

 place in the diagnosis. They are, however, of some interest 

 from a morphological point of view, especially as it is not 

 easy to see how they arose. The sinking of a tegminal 

 plate, on the left of the posterior interradius, so low that it 

 rests on the posterior basal, is no doubt in accordance with 

 the general development of this genus, and tends to counteract 

 the greater narrowness of the cup in the present species ; at 

 the same time it would be rash to regard so unusual a struc- 

 ture as more than an individual variation. 



The relations of the left posterior infrabasal to the posterior 

 basal, though probably obtaining throughout the species, are 

 very remarkable, and more material is required for their com- 

 plete elucidation. It is even possible that, if all future speci- 

 mens were found to possess the many curious characters of 

 the present one, the establishment of a new genus would be 

 warrantable. But under any circumstances the species would 

 remain close to T. callipyyus. Systematically considered 



