990 MR. F. A. BATHER OS triKTACRlITlTS. [DeC, 17, 



in a less degree, to the twisted stem of the Platycrinidse. The 

 advantage which such an arrangement of arm-joints confers on a 

 free-swimming crinoid is obvious, since the animal is thereby 

 enabled to progress more rapidly in any desired direction. 



The origin of this skewing may be connected with the pinnulation 

 of the arm. Bach pinnule-bearing brachial is essentially an 

 axillary. An axillary normally has two joint-surfaces and two 

 fulcral ridges at its distal end ; and these ridges are not parallel 

 to the transverse axis of the ossicle, but converge dorsalwards. 

 As one branch diminishes and becomes a pinnule, the joint- 

 surface on that side also diminishes, while the other joint-surface 

 comes to occupy the greater width of the ossicle, and its ridge 

 becomes parallel with the transverse axis of the ossicle. We may 

 suppose that in Uintacrinus the slanting of the ridge was main- 

 tained, though the ossicle underwent the usual changes. It is of 

 course the case that in the pinnulate arms of other crinoids, e. g. 

 Pentacrinus and Metacrinus, there is an asymmetry of the joint- 

 surface, due to pinnulation, as was long ago well described by 

 Johannes Miiller ^ ; but I can find no instance of a skewing so 

 marked as in Uintacrinus. 



The syzygies are of the type common in the Antedonidce 

 (PL LIV. figs. 4, 5). From the periphery of the brachial clearly 

 defined ridges converge to the axial canal. All the ridges do not 

 reach the axial canal, but only alternate ones, or sometimes one 

 in three. The ridges near the medio-dorsal line are the more 

 marked. The space between the ridges seems to be wider than 

 the ridges themselves ; nevertheless I have been unable to dis- 

 tinguish between the upper and lower surfaces of the joint. 

 Sometimes the ridges are slightly channelled. The figure given 

 by Clark (8) seems incorrect in being so symmetrical, and in 

 the meeting of all the ridges around the axial canal. Schlueter's 

 (4) figures of syzygial surfaces in U. ivestfalicus present a very 

 different appearance, in that they show narrow grooves rather than 

 ridges, which grooves have the same arrangement as the ridges in 

 U. socialis. Both Schlueter's figures represent the upper surfaces 

 of hypozygals, and it might be supposed that the under surfaces of 

 the epizygals were ridged. But, in any case, it is odd that there 

 should be no such grooved appearance in any hypozygals of 

 U. socialis. 



In describing the distribution of syzygies in an arm, it is the 

 custom to reckon as one ossicle the two ossicles that are united 

 by such a joint, and to transfer the term syzygy from the union 

 to the pair of united ossicles. But " to my mind ... it is a 

 custom more honoured in the breach than the observance," and in 

 another place ^ I have given reasons for adopting a method more 

 consistent with both correct terminology and morphological ideas. 



'^ " Ueber den Bau des Pentacrinus caput meduscB," Phys. Abh. Akad. Wiss. 

 Berlin, Jahrg. 1841. See p. 213 and pi. ii. figs. 4 & 13 (1843). 



^ " The Term ' Syzygy ' in the Description of Orinoids," Zool. Anzeig. vol. six. 

 pp. 57-61 (Feb. 3, 1896.) 



