UINTACRINUS: ITS STRUCTURE AND RELATIONS. 21 



the pinnule, and the effect of the structure is well shown by the bent and 

 twisted positions they assume, like those of Actinometra, in contrast to the 

 almost straight and relatively stiff pinnules of the Pentacrinidae and many 

 Antedons. The same structure prevails in the free part of the fixed 

 pinnules, and by this character it can sometimes be seen at what point they 

 probably became free. 



The arrangement of the pinnules is generally as described by Mr. 

 Bather. Several of the proximal pinnules are incorporated into the calyx 

 walls by the upward growth of the supplementary plates of the inter- 

 brachial system. From four or five to ten of the lower plates of these 

 pinnules are thus fixed. In adult individuals there are usually six, rarely 

 eight, of the.se fixed pinnules to each interbrachial area. In the young 

 there are sometimes apparently but four, and in .still younger specimens 

 only two. The intersecundibrach areas contain four fixed pinnules in 

 mature individuals, and two, or sometimes apparently none, in the young. 

 It is difficult to tell just where fixation of these pinnules ends. It is probable 

 that in very young individuals there were none fixed. This would be in 

 accordance with the plan of fixation of arms in Strotocrinits and some other 

 Camerata, and of the pinnules in Rtteocrinus and Gli/ptocrimts, with both of 

 which there is a marked similarity in this respect. 



The first fi.xed pinnule is given off from IIBr.j on the outer side of the 

 i\iy ; tlie next from IIBr4 on the inner side ; the next fVom IIBrg on tlie 

 outer side ; the next from IIBr; on the inner side. The normal succes- 

 sion of the five lower pinnules may be conveniently represented by the 

 numeral 24,578. Above tliis each brachial bears a pinnule on alternate 

 sides, except where there is a syzygy, in which case only the epizygal is 

 pinnule-bearing. There are occasional variations from the above arrange- 

 ment,— e.j/.. No. 78a (PI. v., Fig. 3), and No. 150 (PI. V., Fig. 8),— 

 but it is the general rule. It would seem probable from this that 

 secundibrachs 1-2, 3-4, and 6-7 were primitively syzygial pairs, — 

 the non-pinnulate first, third, and sixth plates being hypozygals. The 

 latter is shown to be the case in specimen No. 78 a. 



The following measurements of the .second fixed pinnule in a large 

 specimen ^vill give a good idea of its proportions : — 



Proximal pinnular . . . length 3.UU mm. . . . width 4.5 nun. 

 10th " ... " .75 " . . . . " 1.25 " 



25th free " near top " .50 " .... " .25 " 



