SAGENOCRINIDAE 223 



heterotomy has become better defined, the ramules being confined so far as known to the inside 

 of the dichptom; they are still irregular in size, not diminishing gradually upward; the larger 

 ones apparently occur about half way up, where they become almost as large as the main arm ; 

 the intervals vary from four to six or more brachials, and the ramules themselves bear 

 secondary ramules, sometimes as many as four, also toward the inside of the dichotom. The 

 primary ramules are large and long, not greatly different in appearance from the ten main 

 ray divisions. A second subdivision of the ray, and the reduction of the ramules to a small 

 size regularly diminishing upward and mostly unbranched, would produce the more regular 

 and definite heterotomy of Devonian and Carboniferous genera such as Dactylocrinus, 

 Wachsmuthicriniis and Synerocrinus. 



The interbrachial structure is of considerable interest, as perhaps leading up to a puzzling 

 condition found in the Carboniferous. It is solid in the lower part of the area, with a single 

 large and well-defined plate; following this come a number of much smaller plates, the change 

 in size being abrupt and not gradual, but at the same time not uniform. And here occurs a 

 singular modification : the plates above the first interbrachial are not regular in size or shape, 

 and from the first range they begin to be more or less isolated and surrounded by perisome ; 

 the area of solid plates diminishes until they are entirely replaced in the upper part by peri- 

 some which passes to the tegmen. This structure varies in the specimens, but is beautifully 

 shown by figure 3a of Plate XX, drawn from a more mature specimen than the others ; and 

 it is to be compared with that of Forbesiocrinns nobilis as hereinafter described. The same 

 structure is repeated in the second and higher axils. The anal area is not perfectly preserved 

 in the upper part ; it is very similar to the others, having one very large plate following the 

 truncate basal, and the second range is differentiated merely by being a little wider and hav- 

 ing a few more plates, as in Sagenocrinus and Forbesiocrinns. The stem is more like that of 

 Sagenocrinus than Cholocrinus. 



Lithocrinus is restricted to the Silurian, and has not been found outside of Gotland. 

 The three species remaining in the original list after the removal of L. obesus have been 

 further reduced by the suppression of L. robustus as a synonym ; another, L. miilleri, must be 

 ignored, as the type and only specimen cannot be found, and Angelin's figure is probably a 

 reconstruction. This leaves L. divaricatus as the only species. 



Lithocrinus divaricatus (Angelin) 

 Plate XX, figs. 1-3 



Forbesiocrinns divaricatus Angelin (in part), Icon. Crin. Sueciae, 1878, p. 9, pi. 21, fig. 21; pi. 28, fig. 3 



(not pi. 26, figs. 6, 6a = Cholocrinus) . 

 Lithocrinus divaricatus, Wachsmuth and Springer, Revision Palaeocrinoidea, pt. 1, 1879, p. 53. — Bather, 



Rep. British Assoc, for 1898 [1899], p. 923. — Springer, Jour. Geology, XIV, 1906, p. 515. 

 Forbesiocrinus robustus Angelin, Icon. Crin. Sueciae, 1878, p. 9, pi. 21, figs. 11, 12. 

 Lithocrinus robustus, Wachsmuth and Springer, Revision Palaeocrinoidea, pt. 1, 1879, p. 53. 



Type of the genus. 



Crown elongate, widest about level of first ramule, spread of calyx from 

 base 1 to 3.3. Rays and their divisions deeply rounded, heavy, widely diverging, 

 tapering slowly. Cross-section above IBr pentagonal. Surface marked with 

 very fine granulose wrinkles. Crown of mature specimen, 52 mm. high by 

 20 mm. wide; base, 6 mm. 



IBB forming a low cylinder above column. BB half as large as RR, slop- 

 ing outward, post. B truncate. Anal x large, rising nearly to the height of the 



