UINTACRINUS: ITS STRUCTURE AND RELATIONS. 69 



I have already shown that the statement of Clark as to the regularity in 

 the arrangement of the interbrachials of U. socialis into a ring of plates en- 

 closing one or two others does not hold good when large collections are 

 examined. There are many specimens of adult U. socialis which have 5, 6, 

 or 7 interbrachials, in all of which — and sometimes in those with 8 or 9 — 

 all the plates " come in contact with plates of other areas," and there is no 

 enclosed plate (PI. V., Fig. 6 ; PI. VI., Fig. 1 ; PL III., Fig. 6). And in the 

 young of Locality No. 2 this is the most common form. The variation in 

 the number and arrangement of the interbrachial plates is enormous. The 

 number among adults ranges from 6 — rarely 5, 4, or 3 — (PI. III., Fig. 6 ; 

 PI. v., Fig. 5 ; PI. VI., Fig. 1) to 22 or 23 (PI. VI., Figs. 5 and 6) ; and among 

 the young from 5 — rarely 3 or 4 — to 9 or 10 (PI. III., Figs. 3, 4, 5; PI. V., 

 Figs. 3, 4). Generally there is one interbrachial abutting on the radials in 

 the first range; but sometimes there are two (PI. VI., Fig. 5), or three 

 (PI. VI., Fig. 2) plates in the first range. Sometimes, though rarely, the 

 first interbrachial passes down between the radials and meets the basals, as 

 in the Rhodocrinidte. Cases of this kind are shown by PI. VI., Fig. 5, and 

 PI. v.. Fig. 4, where the first plate of one interradius rests upon a truncated 

 basal. If our knowledge of the genus were derived from either of these 

 specimens alone, we would not hesitate to put an anal plate in the diagnosis. 

 In another specimen, however, the same thing occurs in three interradii 

 (PI. VI., Fig. 6). These are exceptional cases, and represent only spor- 

 adic variations. * 



The variation in the interbrachial spaces is not only exhibited between 

 different individuals, but is equally conspicuous between different areas of 

 the same individual (PI. V., Fig. 6). Indeed it is rather rare to find 

 among the adults a specimen showing three or more interradii in which the 

 number of plates is the same in all. It is more uniform among the young, 

 as we should expect would be the case. The form and proportions of the 

 plates also differ very materially, both in the same and between different 

 specimens. In some areas there are a large number of small plates 

 (PI. VI., Fig. 2), and in others an equivalent space is filled by a few very 

 large plates (PI. VI., Fig. 1). 



The accompanying text figures of interbrachial areas from some of tlie 



* It is interesting to note that an irregularity of this kind is found anion;,' the Ichthyocrinidiv. In 

 certain Taxocrini exceptionally, and in one species as a rule, a plate is interposed between the radials, and 

 touches the basal in two or more interradii. 



