INVERTEBRATES. 207 



Knowing that the number of arms sometimes varies to some extent, in dif- 

 ferent specimens of the same species of crinoids, we were at first inclined to 

 think the form under consideration might be only a young specimen of A. ro- 

 tundus, but on comparing it carefully with specimens of that species of the 

 same size, we find they possess the usual number of arms (20) in all our speci- 

 mens, and uniformly present the other differences mentioned. 



Locality and position: Same as preceding. 



Actinocrinus asteriscus, M. and W. 



Fig. 15, fig. 8 a, 8 6 and 8 c. 



Actinocrinus asteriscus, Meek and Worthen, Sept., 1860. Proceedings Acad. Nat. Sci., 



Philad., p. 385. 

 Actinocrinus zequibrachiatus, var. alatus, Haxl, iTany., 1861, Boston Jour. Nat. Hist., 



vol. vii, p. 263. 

 Compare A. xquibrachiaiics, McChesney, Jany., 1860. New Palaeozoic Fossils, from 



Western States, p. 25. 



Body rather under medium size, much depressed, the under 

 side being somewhat more convex than the vault; stelliform 

 as seen from below or above, in consequence of the excavation 

 of the interradial spaces; sides expanding rapidly and curving 

 outward, so as to bring the third radials, secondary radials and 

 first brachial pieces all upon a horizontal plane; plates rather 

 thin, apparently subgranulose, flattened, and separated by close 

 fitting sutures. Base small, rounded and destitute of a distinct 

 marginal rim, two and a half to three times as wide as high. 

 First radial plates wider than long, three of them hexagonal 

 and two heptagonal, the angle at the middle of the base of the 

 latter being very obtuse. Second radials small, about twice as 

 wide as high, pentagonal and hexagonal. Third radials some- 

 what larger than the second, pentagonal or occasionally hexa- 

 gonal, about twice as wide as high, and each supporting on its 

 superior (or more properly outer) sloping sides, two secondary 

 radials ; these are each succeeded by another, which in its turn 

 supports two of the brachial pieces, making in all four arms to 

 each ray, or twenty in the entire series. 



