FOSSIL CRINOIDS. 159 



part of arms attached, and another fragment, which Mr. Braun has kindly 

 placed at my disposal for investigation. 



This is a free floating Crinoid, and, like Uintacrinus, has an element in the 

 calyx additional to that usual in the class, viz, a centrale, developed within the 

 ring of infrabasals. In Uintacrinus, as I have elsewhere shown, the infrabasals, 

 as well as the centrale, are small and relatively inconspicuous, the infrabasals 

 in somewhat more than half the specimens not appearing at all; but in Mar- 

 supites these two constitute about one half of the calyx. As to these elements, 

 Marsupites is in the same condition morphologically as the dicyclic form of 

 Uintacrinus, and in fact this is so as to all the structures up to the arm bases, 

 including the stemless character and the very thin plates. On account of the 

 last two facts the two genera were placed in the same family by H. A. Nicholson 

 and P. H. Carpenter, without knowledge of the still more important similarity 

 in the base. In discussing these relations, I dissented from this view, believing 

 Marsupites to be an Inadunate Crinoid, which Uintacrinus palpably is not. 

 The new American species, and some specimens from England showing these 

 parts better than I had before seen, lead me to think the interbrachial plates 

 in this genus of more importance than was before supposed. There is certainly 

 more of a definite structure here than is expressed by the term "loose incorpora- 

 tion of brachials"; the interbrachials appear to be very firm plates, connected 

 with the brachials by straight sutures. The union of brachials with radials in 

 Marsupites is by straight muscular articulation, though it only occupies part 

 of the distal face of the radial, as in Poteriocrinus; in Uintacrinus it is doubtless 

 modified by the incorporation of brachials, and consequent loss of motion in 

 the joint, but farther out, where the arms become free, the muscular articula- 

 tion is resumed. Inadunata with a slight development of true interbrachial 

 structures have now been found in certain Ordovician species, which indicates a 

 close approximation of the two orders, Inadunata and Flexibilia, at a period near 

 their divergence; and it is not unreasonable to suppose that such a condition 

 has recurred in more recent epochs, where the -dominating morphological feature 

 was something else. I cannot think the presence in these two genera of a similar 

 new element in the calyx, unknown in other Crinoids, within the ring of infra- 

 basals, whether primarily derived from the stem or not, was a wholly independent 

 development; and for that reason, together with the other general similarities, 

 it seems probable that there was a nearer relation between them than I before 

 supposed. Mr. Austin H. Clark (Proc. Biol. Soc. Washington, XXII, 174, 



' Uintacrinus. Mem. Mus. Conip. Zool., XXV, Pt. I, 53 (1901). 



