SAGENOCRINIDAE 255 



Types. Hall's original is in the Worthen collection now in the University of Illinois ; 

 that of figure 8 in the University of Chicago ; the others figured are in the author's collection. 



Horizon and locality. Lower Carboniferous, Keokuk limestone beds west of the Illinois 

 Coal Measures ; Keokuk, Iowa, and various localities along the Mississippi River from 

 Nauvoo to the mouth of the Illinois River, associated with Agaricocrinus wortheni, Dory- 

 crimts mississippiensis, and Uperocrinus nashvillae. 



Forbesiocrinus multibrachiatus Lyon and Casseday 

 Plates XXVIII, figs. i-ip; XXIX, figs. 4, 5 



Forbesiocrinus multibrachiatus Lyon and Casseday, Amer. Jour. Sci., XXVIII, 1859, p. 235. — Miller and 



Gurley, Bull. 8, Illinois St. Mus., 1896, p. 55, pi. 4, fig. 1. — Springer, Jour. Geology, XIV, 1906, 



p. 489. 

 Taxocrinus multibrachiatus, Wachsmuth and Springer, Revision Palaeocrinoidea, pt. I, 1879, p. 49. 

 Forbesiocrinus wortheni, Meek and Worthen (not Hall), Geol. Surv. Illinois, V, 1873, p. 496, pi. 14, fig. 2; 



pi. 15, fig. 7- 

 Forbesiocrinus speciosus, Miller and Gurley, 16th Rep. Indiana Geol. Surv. (1888) 1S90, p. 347, pi. 5, 



figs. 8, 9 (Author's ed. 1890, p. 27). 



A medium-sized species very similar to F. wortheni, and perhaps only a 

 variety of it. The relative size, number and proportions of plates, and spread 

 of calyx are substantially the same. In general the calyx is rather more evenly 

 rounded, and cross-section more obtuse; the arms are more deeply rounded and 

 not so closely abutting, there being usually distinct elongated spaces for some 

 distance above the bifurcations. Taking an average of 10 measured specimens, 

 while the height to width of the whole crown as usually found is about the same, 

 1 to 0.7, the calyx to top of iBr is relatively higher, and the visible parts of the 

 arms shorter; the ratio of height to width at greatest width, IIBr 3 , is i to 1.8; 

 that of height of calyx to crown is about i to 1.6, and height to width of calyx 

 1 to 1.2, as against 1 to 2 and 1 to 1.5 for the same ratios in the former species. 

 Dimensions of a maximum and very small crown are : 



Maximum: height of crown, 61 mm.; of calyx, 35; width, 40; base, 13. 



Small: " " " 14 mm.; ". " 9; " 10; " 4. 



This species is found in the Indian Creek crinoid bed at several localities within a short 

 distance along the creek of that name in Montgomery County, Indiana. That bed is in the 

 eastern uplift of the Keokuk Group, separated from the western or Mississippi Keokuk by 

 the Illinois coal field, to the east of which the Keokuk beds are considerably thinner and lose 

 their predominant limestone character, becoming much more impure and argillaceous. By 

 these differences in the waters of sedimentation the crinoid fauna was considerably modified, 

 so that while the general type remains the same the majority of the species are not common 

 to the two areas. For that reason this species may probably be maintained upon the slight 

 differences above shown. The Indian Creek specimens have been found in considerable 

 abundance and in good preservation. They occurred with other genera in small colonies, 

 from one of which I have a fine series representing stages of growth from very young to 

 mature, some of which are figured on Plate XXVIII. These are interesting to show the 

 development of the interbrachial system from one or two plates to 15 or 20, according to the 

 stages of growth. In some of these may also be seen a tendency among the plates of the 

 anal side to develop a vertical series in the midst of the solid plates, with a line of weakness 

 along its left margin. 



