48 UIXTACRINUS: ITS STKUCTURE AND EELATIONS. 



their number is small, and as they diminish rapidly towards the top, we 

 obviously have the entire proboscis." He does not state distinctly what 

 the position of this organ is, but as he assigns the mouth to the middle of 

 the disk, it is clear that this is excentric or lateral. Unfortunately no 

 figure of the specimen is given. 



A specimen of Pentacrmus fossilis from the same locality, in my col- 

 lection, has the tegmen preserved. It is laterally crushed, like the 

 Buckland specimen, but does not show the symmetric form nor central 

 protuberance which that one does. The plates of the disk are well- 

 defined, and join by distinct sutures. It is of very large size, and rises 

 high up between the arms. It shows neither mouth, ambulacra, nor anal 

 tube or opening. 



The living Pentacrinidos, which are the recent representatives of the 

 dicyclic Inadunata, have the mouth central, and the anus excentric, notwith- 

 standing the anal structures have ceased to have any influence upon the 

 composition of the calyx, — there being no distinct anal plates in either cup 

 or tegmen. 



Hyocrbms, which is considered to be the living representative of the 

 monocyclic Inadunata, has a central mouth covered by five oral plates, 

 probably capable of opening; ambulacra with covering plates and side 

 pieces, and anal tube excentric. 



From the foregoing review of the structures of the known tegmens, it is 

 clear that there is no similarity between the disk of Uintucriniis and that of 

 any of the fossil Crinoids, except that in some Fistulata with a very large 

 ventral sac, and in the Triassic genus Holocrimis, the mouth may have been 

 excentric. 



In the recent Stalked Crinoids the construction of the disk varies con- 

 siderably. It may be more or less completely covered by plates,* or it may 

 be entirely bare to the naked eye, although containing microscopic spicules 

 at the sides of the ambulacra.! But in all of them the mouth is situated at 

 or near the centre of the disk,^ and the anus is excentric or marginal; and 

 all of them have a plated ambulacral skeleton. Comparison of the figures 

 of disks of various Pentacrinidae in the Challenger Eeport on the Stalked 

 Crinoids, on Plates XVII., XXVI., XXX., XXXII., XXXIV., XXXIX., 

 XLIII., and L., will show how utterly different is the tegmen of Uintacrinus 

 from any of them. 



* Challenger Report Stalked Crinoids, p. fi7. f Ibid., p. 68. % Ibid., p. 68. 



