CRINOID GENUS SCYPHOCRINUS 53 



the second range. Secundibrachs about 13 to 15, with median ridge rather 

 narrow, commencing at about the sixth or eighth plate; fixed pinnules fre- 

 quently distinguishable in regular order. Plates thick, not pustulose; broadly 

 convex in lower part, with strong stellate sculpture above, produced by rather 

 broad connecting ridges and pits. Arms apparently rather slender. 



Dimensions of a large specimen : Height of calyx, 70 mm. ; width at widest 

 interbrachial zone 57 mm., reduced directly above to 50 mm., beyond which it 

 expands again to the arm-bases; diameter of stem at calyx, 9 mm. 



This is a thoroughly well-marked and abundant species, of the type of 

 S. pratteni, but differing consistently, and from a different horizon. At 

 Pyburn's Bluff, on the Tennessee River, which is its typical locality, pratteni 

 does not occur at all, while at the four localities where that species is the 

 principal crinoid, pybumensis was not found except for a single specimen in 

 the talus, clearly derived from higher beds. The material in hand consists of 

 35 specimens, many of which have one or more rays intact to the bifurcation 

 above the secundibrachs. In one an arm is preserved for two branchings, 

 much more slender than those of S. elegans. All the specimens are readily 

 distinguished from S. pratteni by the more elongate calyx, less prominent 

 protuberance of the interbrachial areas, greater asymmetry of the posterior 

 interradius, and lack of the spinose center on the plates. In very elongate 

 specimens, such as that of Plate VII, figures 2a, b, the secundibrachs retain the 

 elongate form and general aspect of calyx plates to the height of seven or 

 eight plates before passing into the median ridge; above that they become 

 very short and wide. There is not much variation in size, specimens being 

 rarely less than 50 mm. high, unless the few which I have mentioned under 

 S. gibbosus should prove to be the young of this species. 



The fixed pinnules are better defined in this than in any of the other 

 Tennessee species. I have figured two specimens in which their course is 

 perfectly plain (PI. VII, fig. 2b; PL VIII, fig. 6a). The latter of these exhibits 

 an interesting abnormality in which the first interbrachial on the posterior side 

 passes down to the basal, producing an anal area similar to that of the Actino- 

 crinidas (PI. VIII, fig. 6b). The case is the converse of that of the abnormal 

 Teleiocrinus mentioned by Wachsmuth and Springer * in which the anal plate 

 between the radials is lacking, as in the Melocrinidae. 



In the matter of surface sculpture in this and other species found in the 

 Ross limestone, the observer must be on his guard against the effects of 

 chemical action and weathering, by which the aspect of the plates is sometimes 

 completely altered. Often the original calcareous surface is destroyed, while 

 certain structures, such as the channels, or ridges connecting the plates, are 

 replaced by silica, leaving an abrupt and rugose ornament totally different 



1 The North American Crinoidea Camerata, Mem. Mus. Comp. Zool. Harvard, vol. 20, p. 56, 1807. 



