242 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



the other plates. Anal opening almost central. Column round, filling 

 nearly the whole width of the basal concavity ; axial canal small and pent- 

 angular. 



Horizon and Locality. — Highest part of the Upper Burlington limestone, 

 Burhngton and Pleasant Grove, Iowa. Rare. 



Type in the Museum of Comparative Zoology, Cambridge. 



Remarlis, — In this and the two preceding species, which were the last sur- 

 vivors of the genus, the individual tubes are constructed of three rows 

 of plates instead of a single one as in all others. This species, however, 

 differs essentially from all others in the form of the calyx. While in G, 

 tiiberosiis and similar forms the basals form the sides of the lower concavity, 

 they are placed in G. obovatiis at the sides of the dorsal cup, which is 

 decidedly convex in the latter, but flat or even concave in the former. 



Gilbertsocrinns typus (Hall). 

 Plate XIV, Figs, i, 2, 3, and Plate XVII, Figs. 7a, K 



1859. Trematocrinus typus — Hall; Suppl. Geol. Rep. Iowa, p. 73. 



1873. Gilbertsocrinus {Trematocrinus) typus— B.al1j; Bull. Museum Nat. Hist., Plate 6, Eig. 13. 

 1873. Goniasteroidocrinus typus — Meek and W^ortiien ; Geol. Rep. Illinois, Vol. Y., p. 890. 

 1881. Ollacrinus typus — W. and Sp. ; Revision Palseocr., Pt. II., p. 219. 

 1889. Goniasteroidocrinus typus — S. A. Miller ; North Amer. Geol. and Palfeont., p. 250. 

 Syn. Trematocrinus fapillatus'KKL-h\ Suppl. Geol. Rep. Iowa, Yol. I, p. 76. 



Calyx broadly concave at the base, somewhat inflated at the middle, 

 contracted near the top, spreading above into a projecting rim around the 

 upper margin. Ventral disk low-hemispherical or slightly convex. Basals, 

 radials, and interradials produced into sharp central spines or elongate 

 nodes; all other plates convex and slightly tumid. 



Infrabasals comparatively large, forming a pentagon, of which the greater 

 part is exposed beyond the column. Basals very large, curving so that the 

 lower halves of the plates rest within the columnar concavity, the upper 

 forming a part of the lateral walls of the dorsal cup; their spines directed 

 downward, while those of the other plates are directed outward. Eadials 

 much larger than the costals ; the interradials one half smaller, but larger 

 than the interbrachials. First distichal smaller than the second, the latter 

 higher and axillary, supporting two arms, which branch from the third 

 palmar, and again on one side from the third post-palmar, making a cluster 

 of arms to each opening. Arms pendent, long and delicate, uniserial to the 

 last bifurcation, when the plates become cuneate and interlock. Pinnules 



