246 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



plates in the third, fourth, and fifth rows, and two small plates above. Inter- 

 distichal areas composed of a rather large plate^ succeeded by four or five 

 smaller ones. Ventral disk almost flat, with deep interradial depressions. 

 Some of the plates are larger and more convex, but none of them are refer- 

 able to orals. Anus subcentral, on top of a small protuberance. Column 

 unknown. 



Horizon aud Locality. — Lower Burlington limestone ; Burlington, Iowa. 



Type in the Museum of Comparative Zoology, Cambridge. 



Gilbert socrinus tenuiradiatus (M. and W.). 

 Plate XVII. Fig. 3. 



1869, Goniasieroidocrinus tenuiradiatus — Meek and Worthen; Proceed. Acad. Nat. Sci. Phila., p. 75. 

 1873. Goniasteroidocrinm tenuiradiatus — Meek and Worthen ; Geol .Eep. Illinois, Yol. Y., p. 389, Plate 



11, Fig. 1. 

 1881. OUacrinus tenuiradiatus — W. and Sp. ; Revision Palseocr., Part II., p. 219. 

 1889. Goniasteroidocrinus tenuiradiatus — S. A. Miller; North Amer. Geol. and Palseont., p. 250. 



The two specimens before us are considerably crushed, and their general 

 form and the arrangement of the plates cannot be accurately determined. 

 It is a larger species than the two preceding ones, wdth which it is found 

 associated, and which it resembles in the delicacy of the plates ; but in other 

 respects it is more like G. tyjpus^ of a higher horizon. The surface of the 

 plates is marked by a series of elevations, radiating from the middle of the 

 plates to adjoining ones, which, however, are not ornamented ridges, but are 

 produced by a folding of the plates. The basal concavity is quite shallow, 

 and composed almost exclusively of the infrabasals. 



Basals large, curving upwards and inwards, their upper lateral faces 

 longer than the corresponding lower ones \ they are extended into a sharp, 

 slender spine rising from the centre of the plates, unlike the case of Q. typus, 

 in which the spines cover the whole surface of the plate. Radials very 

 large, and mounted with similar spines as the basals. Arms given off from 

 the second distichals ; their structure unknown. Calycine appendages very 

 long, and tapering but slightly; their joints are strictly cylindrical and 

 devoid of ornamentation ; they are arranged interradially in pairs, and thope 

 of each pair are connected laterally by zigzag sutures to the fourth or fifth 

 joint, when they become free and diverge in opposite directions. The 

 number of arms and the number and arrangement of interradial and inter- 

 distichal plates cannot be ascertained in the specimens. 



