ACTINOCRINID^. 629 



over the sides of the two or three proximal stem joints; the suture hues 

 deeply notched, the grooves reaching to the top of the plates. Eadials as 

 long as wide, and as large as both costals together ; the second costals 

 smaller than the first. Distichals as large as the axillary costals; the suc- 

 ceeding brachials much smaller, giving off the arms in the usual way. Large 

 specimens have seven bifurcations in each main division, and hence sixteen 

 arms to the ray, smaller ones five to six, with twelve to fourteen arms. 

 Arras of moderate length and quite delicate, their lateral margins serrated. 

 Regular interbrachials : 1, 2, 2, 2, gradually decreasing in size upwards. 

 Anal plate generally a little smaller than the radials, followed by 2, 4, 3 and 

 2 plates. Ventral disk almost flat at the margin, dome-shaped above. It 

 is composed of large and small plates, the former nodose, the latter convex 

 and interposed between the larger ones. Anal tube central, stout and long, 

 rising above the tips of the arms, and constructed of rather small, convex, 

 transversely elongate pieces. Column of less than medium size ; the nodal 

 joints long, their outer margins crenulated, being covered with numerous 

 small, longitudinal processes, which hang down slightly over the intervening 

 smaller joints. 



Horizon and Locality. — Upper Burlington limestone, Burlington, Iowa, 

 Quincy, 111., and other places. 



Type in the (Worthen) Illinois State collection. 



Bemarlis. — We have examined of this species over one hundred speci- 

 mens of all sizes, most of them calyces, but some with the arms attached, 

 and find among them considerable variation in the ornamentation (compare 

 Plate LX. Fig. 2a with 25), as well as in the number of arm openings, 

 without showing any other structural differences; they even agree in the 

 peculiar and unique ornamentation of the stem. A careful comparison of 

 these specimens has shown us no way by which a separation of them can be 

 made upon any constant characters. "We have observed that the 3'ounger 

 specimens have fewer arms than the older ones, and that the number of arm 

 openings increased as the rim grew larger, i. e., extended out farther. We 

 also found that the specimens in their earlier phases passed through the 

 Caciocrinus stage, where they had no rim, and in which some of the higher 

 bifurcations took place in the free arms (Plate LX., Fig. 2cl). Among the 

 calyces, the smallest ones have but 4 arm openings to the ray, somewhat 

 larger ones 6, others 8, 10, 12, or 14, while in the largest ones there are 16; 

 showing again how little reliance, in some groups, can be placed upon the 



