FOSSIL CRINOIDS. 131 



radials, or else a very large one, axillary, and bearing two rays having upwards 

 of twenty ultimate divisions to each. The other specimens from Louisville, 

 PI. I, fig. 2, and PI. II, fig. 1, and two others showing arm structures not figured, 

 were collected by Mr. Geo. K. Greene, from whom I acquired them. 

 Horizon and Locality. As above stated. 



Arachnocrinus knappi Wachsmuth and Springer. 

 Plate I, figs. 5, G, 7. 



1879. Rev. Pal., I, 93. 



A large species, similar to A. extensus in the non-differentiaticn of the axillary 

 brachials (and perhaps only a variant of it), but differing from it in the decidedly 

 greater number of primibrachs in all the rays. It was founded upon a unique 

 specimen, and no others have been discovered; this has four rays preserved to 

 the first, and partly to the second, bifurcation, the right posterior one being lost; 

 these have brachials as follows: — 



1. ant. 1. post. r. post. r. ant. ant. 



IBr. 11 14 — 9 14 



IIBr. 3-4 — — 4-4 5 



The right anterior ray, by oversight called the right posterior in the original 

 description, is much smaller than the others (as rather poorly shown in Fig. 5, 

 PL I), and was evidently recuperated, so the average number of IBr should be 

 taken from the other three, as about 13. 



There are enough minor differences to make one think that this great diver- 

 gence in the condition of the rays, as contrasted with the almost uniform 3 IBr 

 in all but the anterior ray of A. extensus, is not merely sporadic. In this particu- 

 lar it is more like A . bulbosus, while wholly unlike it otherwise. The primibrachs 

 are short and wide — 1 to 3.6 — and their surface is transversely convex, and 

 depressed at the sutures, so that the arm dorsally resembles a series of inter- 

 rupted ridges or rings, which is not the case with any of the specimens of ^4. 

 extensus. The radial facets are large, occupying a good part of the plates, and 

 facing obliquely upward. The calyx is well preserved; base concave, with infra- 

 basal disk at the bottom of the fairly large concavity, where details of its condi- 

 tion cannot be seen; basals large, rather tumid, extending well into the cavity. 

 Anal opening very large, considerably excavated into the posterior basal, and 

 succeeded by an elongate triangular plate just reaching to the upper edge of 

 the radials. 



