1032 DR. J. W. GREGORY ON THE CLASSIFICATION _[ 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 each segment. The 
correct explanation appears to be that the smaller pieces are only 
triangular, distal portions of the ambulacral ossicles, apparently 
separated from the proximal portion by a groove. The ambulacral 
plates of Chetroptaster giganteus, Stiirtz [16. pl. 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 
Paleozoic 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. 
PROTASTER BIFORIS*, n. sp. (Figs. 2, 3, p. 1033.) 
Diagnosis.—Dise 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 into 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 attached and form a regular series of 
marginal plates. 
Arms very flexible. 
Dimensions : 
Length of longest arm. Jic4.-.G\s Sees 18 mm.+4. 
Diameter of arm at the base ............ 5 ee 
ne arm near the distal end ...... if A 
A MGUUN .35-< ears en ere + ieee: 
Hheiethrofsynenath |. .7\2.ee canner? oe he tee 
Width of ambulacral furrow at edge of disc. 1°25 ,, 
Distribution Wenlock Shale. Castell Dinas, Bran, near 
Llangollen. Mus. Pract. Geol. No. VI. |;&. 
Affinities —This species of Protaster differs from P. sedgwicki, 
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 muscles. Thereby the arms could be 
rolled up ventrally and the ambulacral furrow thus protected. 
1 Biforis, having two holes or openings; a character due to the hole-like 
appearance of the ventral muscle-pits. 
“Syngnath,” the united piece formed of mouth-frame and jaw. The “Mund- 
ecksteck.” 
