50 UIXTACEIXUS : ITS STKUCTUKE AND EELATIONS. 



Chall. Eep. St. Crin. PI. LV., Fig. 2, and I reproduce Carpenter's figure 

 of it for comparison (PI. IV., Fig. 6). 



The di.sk is composed of a more or less irregularly plated integument ; 

 it is in some specie.s very completely plated, tliougli it may be entirely 

 membranous in the same species elsewhere.* 



For further illustrations of the disk of Adinometra see Challen2;er 

 Eeport on the Comatula^, Plates LYII., LXII., LXIV., and LXVIII. 



Comparison of the foregoing description and figures of Adinometra with 

 those of Uintacrinus hereinbefore given, leaves no doubt that in all essen- 

 tial features the disks of the two forms are identical. The resemblance 

 between Adinometm and Uiiilacnnus is not confined to the disk. Both are 

 remarkable for the absence of any ambulacral skeleton in the arms ; the 

 food grooves in both are simply open, shallow grooves, without j^hated 

 covering of any kind. 



A complete calcareous plating at the sides of the ambulacra on the 

 arms and pinnules is a constant characteristic of Antcdon ana the Pen- 

 tacrinidte, but it does not exist in Actinometra. P. H. Carpenter says 

 concerning it : f "The entire absence of side plates and covering plates 

 in the arms and pinnules of Actinometra, even in species which have a 

 strongly plated disk, is a very singular peculiarity, and one which I am 

 quite unable to explain." 



Besides this, there is a striking similarity in the whole aspect and 

 structure of both arms and pinnules. The distribution of syzygies is similar. 

 The shape and proportions of the brachials — bearing in mind that those 

 of Uintacrimis are always flattened by pressure — are substantially the same. 

 The superficial resemblance of the pinnules of Uintacrinus to those of 

 Adinometra is remarkable. If any one will compare the pinnules as they 

 appear in PI. V., Fig. 1 ; PI. VII., Fig. 4, with those of the various Adino- 

 metrce figured in tlie Challenger Report on the Comatula^, on Plates LTII. 

 to LXVIII., he cannot fail to be impressed by the great similarity. It is 

 even more striking if one compares the pinnules of Uintacrinus as they 

 lie on the slabs (PI. VIII., Figs. C 3 and B 7), in some of the most perfect 

 specimens, with those of alcoholic specimens of Adimmdra. The pecu- 

 liar shape, the bent and twisted positions they assume, the relative 

 proportions in different parts of the arm, the deep serration of the proxi- 



« Chall, Rep, ComatulfE, p. 276. 



t Bull. Mus, Comp. Zobl, Harvard Coll., IX., No. 4, p. 13. 



