1032 DB. J. W. QEBGOEY ON THE OLASBIFICATION [Dec. 15, 



Forbes's figure represents the ambulacral ossicles as being 

 alternately large and small. It is quite possible that each small 

 pair represents a segment, and that the smaller ossicles have been 

 reduced by absorption in order to give space for the podia. This 

 explanation, however, seems improbable ; for in that case there 

 would be only one podion, instead of a pair, to eacii segment. The 

 correct explanation appears to be that the smaller ]iiecos are only 

 triangular, distal portions of the ambulacral ossicles, apparently 

 separated from the proximal portion by a groove. The ambulacral 

 plates of Cheiroptaster giganteus, Stiirtz [16. pi. xxx.], for example, 

 are forked, and the junction is depressed ; if we only knew this 

 genus from internal casts, it would appear that the two prongs of 

 the forked piece were separate. Many old figures represent 

 Palaeozoic Ophiurids as having the ambulacral ossicles alternately 

 large and small; but it is quite possible that the explanation 

 suggested will account for all such cases. 



Pbotastee bifokis \ n. sp. (Figs. 2, 3, p. 1033.) 

 Diagnosis. — Disc fairly large ; interbrachial outlines concave. 

 The syngnaths ^ are simple, prominent, and stout. The ambulacral 

 ossicles consist of a thick body and a stout curved wing. The 

 distal margin of the ossicles is notched by a depression for a 

 ventral muscle-field, which also cuts iuto the proximal margin of 

 the adjoining ossicle. Owing to these muscular depressions the 

 arm has apparently two series of pores. 



The adambulacral ossicles are massive and taper slightly to their 

 distal ends ; they are closely attaclied and form a regular series of 

 marginal plates. 

 Arms very flexible. 

 Dimensions : 



Length of longest arm 18 him. + x. 



Diameter of arm at the base 2-5 „ 



„ arm near the distal end 1 „ 



„ mouth 1'5 „ 



Length of syngnath 1 „ 



Width of ambulacral furrow at edge of disc. 1-25 „ 

 Distribution. — Wenlock Shale. Castell Dinas, Bran, near 

 Llangollen. Mus. Tract. Gcol. No. VI. | yV 



Affmiiies. — This species of Protasier differs from P. scdgwicM, 

 Forbes, in having concave interbrachial outlines and roughly 

 triangular adambulacral ossicles, and also by the character of the 

 ambulacral ossicles. The species is of interest as showing that 

 the flexibility of the arms is due to the great development of some 

 ventral inter-ambulacral nniscles. Thereby the arms could be 

 rolled up ventrally and the ambidacral furrow thus protected. 



' Bifoi-is, having two holes or openings; a elinracter duo to tlio hole-lito 

 appearance of the ventral muBole-pilB. 



^ " Syngnath," the united piece fornied of mouth-frnnae and jaw. The "Mund- 

 ecksteck." 



