n4 PROCEEDINGS OP THE ACADEMY OF [1886. 



TRIBRACHIOCRINUS McCoy. 



1847. McCoy, Ann. and Mag. Nat. Hist., xx, p. 228. 

 1859. Pict^t, Traite de Paleont., vol. iv, p. 321. 



1884. F. Ratte, Proceed. Linnsean Soc. of New South Wales, vol. ix, 

 Part 4. 



(Diagram, PI. 6, fig. 5). 



This genus was originally proposed for a single specimen from 

 the Subcarboniferous of Australia. According to McCoy, it 

 differs from Gyathocrinus and allied genera by having only three 

 arms. The cup is described as cupuliform, large, composed of 

 three plates in the proximal ring, 5 plates in the succeeding one, 

 followed by 1 X 3 radials, 3 interradials, and one or two anal 

 plates. A similar explanation of the plates has been given lately 

 by Mr. Ratte, who described another species in which " the second 

 radials," the brachials according to our terminology, were pre- 

 served. Mr. Ratte had the kindness to send us an excellent cast 

 of the type specimen, examination of which leads us to infer that 

 the third ring of plates in the calyx was composed of seven 

 pieces, of which five were radials, the two others azygous plates, 

 but that none of them are interradials. In three of the radials, 

 the articulating faces form a straight horizontal line, and only 

 these plates are apposed by regular brachials, the two others, 

 those of the two antero-lateral rays being angular and higher at 

 their distal ends. The general outline of the two last mentioned 

 plates, indicates that they are compound plates, each representing 

 a radial and a bifurcating brachial, which probably became 

 anchylosed. They evidently supported two arms, one at each 

 side ; while the three radials with articulated brachials apparently 

 bear but a single arm like Gr-omyocriyius simplex Trautchold. 

 Ratte observed in Tribachiocrinus corrugatiis along the ventral 

 surface between the brachials, " the casts of very small plates, 

 irregular in shape, which doubtless belong to the so-called voute 

 (vault), or outer part covering the calyx as in Bhodocrinus, for 

 instance." We seriously doubt if these plates, which he figures 

 on PI. 68, fig. 2, are any such thing as vault plates ; we believe if 

 they are plates at all, that they formed a part of the disk, and as 

 such were covering pieces. The radials enclose the azygous plate 

 proper and an anal piece as in most of the Poteriocrinidse, 

 Tribachiocrinus is not such an aberrant genus, as it was supposed 

 to be. It is closely allied to Cromyocrinus and Agassizocrinus, 



