1879.] NATURAL SCIENCES OF PHILADELPHIA. 279 



that organ having been discovered, though we had occasion to 

 examine on this point some remarkably perfect specimens. 



Revised Generic Diagnosis. Body, with arms included, short ; 

 the fivg nays equilateral ; arms divergent. Plates of calyx un- 

 usually thin, and, as far as the top of the first radials, forming a 

 basin or depressed cup composed of immovable plates. Succeed- 

 ing plates, though connected laterally, movable. 



Underbasals small, scarcely extending beyond the column. 

 Basals much smaller than the first radials, all pentangular and 

 of equal size. Radials three to four by five. First radials com- 

 paratively much larger than in any other genus of this family ; 

 articulating face concave, and occupying less than one-half the 

 width of the plate; the upper margins on both sides of the scar 

 nearly straight, almost horizontal, scarcely inflected, and support- 

 ing the interradial portions which are rarely preserved, leaving 

 in their place, in the fossil, a wide, open space between the rays. 

 Second and third radials short, often three or four times wider 

 than high, lunulate, resting in the concavity of the preceding 

 plate; the bifurcating plate almost triangular. Succeeding order 

 of plates constructed like those of the arm, only larger, all of 

 them much wider than high, rounded on the back, with distinct 

 waving sutures showing deep sinuosities when the arms are 

 closed, and indicating a great mobility in these parts. Arms 

 divergent, tapering gradually ; pinnulae unknown. No interra- 

 dials to the top of the first radials, but the succeeding radials, and 

 probably the first plate of the next order, connected by an inter- 

 radial integument, the true nature of which has not been fully 

 ascertained, being probably similar to that in Taxocrinus, but 

 composed of larger plates. Column round, thick, composed near 

 the calyx of narrow joints. 



Only two species are known, and both are from the Burlington 

 limestone : 



1873. Nipterocrinus arboreus Wortheh. Geol. Rep. Illinois, vol. v. p. 436, pi. 4, 

 fig. 8. Lower Burlington limest. Burlington, Iowa. 



1868. Nipterocrinus Wachsmuthi Meek and Worth. (Type of the genus.) Pro- 

 ceed. Acad. Nat. Sci. Phila., p 34] ; also Geol. Rep. Illinois, vol. v. p. 435, 

 pi. 2, fig. 4. Upper Burlington limest. Burlington, Iowa. 



