TAXOCRINIDATC 423 



rami above the fork, it forms the beginning of a series in which the length of the rami 

 progressively increases until they equal and then greatly exceed the main ray. And it differs 

 from them all in the unusual feature of having the ramules branch only to the outside — 

 a structure which finds a parallel in the highly specialized Camerate species Dichocrinus 

 polydactylus. It is otherwise well-distinguished from the other species occurring in the 

 same horizon, 0. diversus, by its 3 IBr and small, non-divergent rami. 



A fact of individual growth which runs through the Onychocrini, to be noted in this 

 form, is the progressive joining of the rays by interlocking of IIBr above the axillary. In 

 young specimens they separate at once ; then the first pair of IIBr are united, and so on 

 until in mature specimens they are joined beyond the first ramule, and to the fourth or fifth 

 pairs of brachials. 



Types. One of Hall's originals (PI. LXIV, fig. 1), formerly in the collection of 

 Dr. C. A. White, is in the author's collection; the other (fig. 2), formerly in the collection 

 of Rev. W. H. Barris, is in the Museum of Comparative Zoology at Harvard. The others 

 figured are in the author's collection. 



Horizon and locality. Lower Carboniferous, Upper Burlington limestone; Burlington, 

 Iowa. 



Onychocrinus diversus Meek and Worthen 



Plates LXV, figs, i-j ; LXVII, figs, na, b 



Onychocrinus diversus Meek and Worthen, Proc. Acad. Nat. Sci. Philadelphia, (1) XVIII, 1866, p. 256; 

 Geol. Surv. Illinois, II, 1866, p. 243; ibid., Ill, 1868, p. 492, pi. 17, figs. 5a, b, and text diagram. — 

 Wachsmuth and Springer, Revision Palaeocrinoidea, I, 1879, p. 54; N. A. Crinoidea Camerata, 

 1897, P- 123. 



A large-sized species. Crown short and broad; and with rays extended 

 forming an irregular five-rayed star bilaterally differentiated by the anal inter- 

 radius. Calyx comparatively small and depressed. Rays large, the part below 

 the fork nearly twice as long as the rami above it; deeply rounded; free from 

 about the second primibrach, and continuing about the same size from there to 

 the second or third pair of secundibrachs. Rami strong, short, widely diverg- 

 ing, tapering to small ends. Ramules bilateral, branching in strong clusters 

 alternately from both sides of the dichotom at intervals usually of 3 brachials ; 

 number of clusters rarely exceeding 5. Sutures but little arcuate, and in 

 slightly weathered specimens appearing nearly straight on the main rays, but 

 more curved above the division. Interbrachials well-developed, with strong 

 plates in the first one or two ranges, followed by gradually diminishing plates 

 curving over to a connection with the tegmen; leaving a broadly curved, cres- 

 centic margin connecting the rays and followed by perisome. Surface smooth, 

 or finely granulose. 



IBB low, visible above column, resembling a columnal. BB in form of 

 large pentagons, with post. B much the larger, rising high between RR. 

 RR and IBr of similar form, very thick, wider than long, diminishing gradu- 

 ally in length. IBr 4. IIBr 3 (exceptionally 4), interlocking in pairs, with 

 somewhat zig-zag sutures (in young specimens less than the full number inter- 

 lock) forming extensions of the ray of about the same width as the IBr ; rays 

 diverging about third pair of IIBr at a wide angle, more than 90 degrees, 



