52 UmiACRINUS: ITS STRUCTUEE A:ND EELATIONS. 



variation in Uintacrims, from the pentagonal centrale to the stellate 

 infrabasal ring, may well invite comparison. 



It is also to be observed that in all these varieties of Actinometra the 

 angles of the centrodorsal are interradial, indicating the actual or potential 

 existence, at some stage of growth, of infrabasals next above them. 



To sum up the facts on this point, we find an identity of structure, or a 

 striking resemblance, between Uintacrinus and Actinometra in the following 

 particulars : — 



1. Excentric position of the mouth, 



2. Central position of the anus. 



3. Absence of any calcified ambulacral skeleton on disk, arms, or 



pinnules. 



4. Structure and distribution of the disk ambulacra. 



5. Form and proportions of brachials, and distribution of syzygies. 



6. Form, size, and general appearance of the pinnules. 



7. Variable size of the anal tube. 



8. Instability of the base. 



SYSTEMATIC RELATIONS OF UINTACEINUS. 



Mr. Bather, in his discussion of this genus, ^ finds its affinities to be clos- 

 est with the pseudomonocyclic Dadocrmus, from the Trias, and thinks it was 

 derived from that or some allied form. He says : '^ We have to choose be- 

 tween monocyclic and pseudomonocyclic forms ; since, had the immediate 

 progenitors of Uintacrinus well-developed infrabasals, one must suppose 

 that these would have been retained and utilized to expand the walls of 

 the cup, as in Marsupites.'' Whether this conclusion be correct or not, it 

 seems to me that the argument by which it was reached will have to be 

 reconsidered. It first essays, in comparing Uintacrinus with other Crinoids, 

 to clear away secondary, or non-essential features, such as the unstalked or 

 free-swimming character, by which H. A. Nicholson and P. H. Carpenter had 

 — erroneously, as Mr. Bather thinks — placed it in a family with Marsupites, 

 This character he believes to be due rather to similarity of environment 

 than to similarity of descent, and he takes the resemblance to be physi- 



* Proc. Zool. Soc. London, 1895, pp. 999 et seq. 



