Igg PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxvi. 



There can be no longer any doubt that the Pentacrinidse are all 

 either actually or potentially dicyclic, though in some species the in- 

 frabasals are resorbed at an early stage. This has been shown by 

 Mr. Clark to be the case in Isocrinus paiTfv {oUtyi niillleri) ,°^ and the 

 observations of P. H. Carpenter (Stalked Crinoids, pp. 292-93) would 

 seem to indicate that a similar condition prevails in /. wyville- 

 thomsoni^ I. asteria, and /. alternicirrus. I have found the same 

 thing to be the case also in certain species of the palaeozoic genus 

 Ichthyocrinus. 



As to specific relations, it is impossible, w^ith the material avail- 

 able, to make any very satisfactory comparison with European 

 species, a great many of which have been described from isolated 

 stem joints. Although the stem as a whole often affords valuable 

 characters for distinguishing species among the recent crinoids, and 

 even a part of it, if the same parts can be compared, little reliance 

 can be placed in species whose identification depends wholly upon 

 the form and articular markings of joints whose position in the stem 

 can not possibly be known. This has been pointed out by Carpenter 

 (Stalked Crinoids, pp. 226, 298), and the fact is well shown by his 

 Plate 22, where many different forms of columnals from the stem 

 of /. wyviUe-thommni are figured. Mr. Clark has recently found 

 by dissection of the stem of a young /. decorus ^ that in the different 

 parts of the same stem may be found almost every type of articular 

 face, from stellate to round, and from a bifascial articulation with 

 transverse ridge as in Rhhocrimis., to the radiating petaloid sectors 

 of the usual Isocrinus type. Several different forms of stem joint 

 are found in the present species, the more common being obtusely 

 pentagonal, while the younger joints near the calyx become stellate. 

 The proximal face of the nodal joint also shows a sharply stellate 

 outline, due to the indentation by the cirrus sockets (Plate 4, figs. 

 9, 10, 11, 12, 13). In the associated material are thousands of 

 separate joints, besides several considerable portions of stems intact, 

 and there is a general uniformity of size and appearance among them 

 which indicates their probable derivation from a single species. 

 They are uniformly different from the much larger ones on which 

 P. asterisois was founded, and from the Utah specimen referred by 

 Doctor White to P. asteriscus,^ but afterw^ards separated from it by 

 Dr. W. B. Clark under the name Pentacrimis whitei, because of its 

 alternating joints. Clark's comparison was made chiefly with the Red 

 Buttes specimens of P. asteriscus ( ? ) , but the separation is doubtless 

 well founded, nevertheless, as the character on which he bases it is 

 clear in his specimen, and can not be shown in the type of P. asteris- 

 cus. The difference between the stem of our species and that of P. 



oProc. U. S. Nat. Mus., XXXV, 1908, p. 87. 



^ Idem, XXXV, p. 88. 



'^ Bull. U. S. Geol. Surv., No. 97, p. 27. 



