202 SMITHSONIAN INSTITUTION 



( i ) The column : this, while cylindrical in both and without any marked enlargement 

 next to the calyx, has in Nipterocrinus proximally very thin knife-edged columnals, which 

 gradually increase in length downward until they become longer than wide, but without any 

 alternation in length or width, thus making the stem smooth throughout ; whereas in 

 Pyciiosaccus the entire stem, so far as known, is composed of large convex columnals with 

 one or more short and inconspicuous ones interpolated. These features in Nipterocrinus 

 are well shown by the fine specimen figured on Plate XV, figure 5, with a complete stem 

 which has plain traces of radicular cirri and a branched root. 



(2) The base : the infrabasals are confined to a low undivided disk, differing therein 

 not only from Pycnosaccus but from all other crinoids of this order, this being the only genus 

 except Mespilocriuus to show any departure from the normal division of infrabasals into 

 three plates. This structure has not hitherto been understood. Meek and Worthen, in describ- 

 ing the genus (Proc. Acad. Nat. Sci., Philadelphia, vol. 20, p. 341), supposed it had five 

 infrabasals, while Wachsmuth and Springer (Rev. Pal., pt. 1, p. 55) were inclined to think it 

 had three ; but neither undertook to give the number with certainty, as the few specimens then 

 known did not show it. Two specimens since obtained (PI. XV, figs. 4, iorf) prove tht 

 actual construction beyond a doubt, especially figure 4, in which the infrabasal disk is isolated 

 and shown to be perfectly solid. 



The systematic position of Nipterocrinus was at first a matter of doubt. Its great super- 

 ficial resemblance to Cyathocrinus scarcely left room for a suspicion that it could belong to a 

 group of which the utterly different Ichthyocrinus was the type, and it was accordingly 

 thought by Wachsmuth and by Meek and Worthen to be probably allied to the former genus, 

 although the sinuous sutures in the rays did not escape their attention, and led them to sug- 

 gest the resemblance in this respect to Taxocrinus and allied forms. As the peculiar charac- 

 ters of the present group came to be better appreciated, it was seen that the last mentioned 

 structure was of more weight than the superficial resemblance, and it is a striking example of 

 Wachsmuth's great sagacity in the interpretation of fossil crinoids, that he inferred the 

 necessary presence of an integument of interradial plates connecting the lower brachials by 

 reason of which it was ranged under the Ichthyocrinidae by Wachsmuth and Springer in 

 1879. 



This supposition was without direct evidence at the time, for the unique specimen figured 

 on Plate XV, figures loa-d, by which it is now fully confirmed, had not then been discovered. 

 This specimen, which is the only one ever found preserving these structures in perfect condi- 

 tion, shows the small plates completely filling the interbrachial spaces to the third order, and 

 leaves no doubt as to the true relations of the genus. It is rather remarkable that in so small a 

 specimen the perisome should appear so far up the arms, indicating an unusually high tegmen, 

 but this is actually the case ; the preservation is so perfect on one side of the specimen that the 

 perisome can be plainly seen firmly soldered to the sides of the arms to the height shown in 

 the figure. Just how the integument rests upon the shoulders of the radials is finely shown 

 in figure 10c from an interradius where the small parts are partly broken away. I have also 

 found traces of these small plates in two other specimens. 



Nipterocrinus has been ignored by Ouenstedt, and by Von Zittel in the Grundziige, owing 

 no doubt to the element of uncertainty in the descriptions of its structure, now dispelled by 

 the discovery of new material. The stratigraphic range of the genus is limited to the two 

 divisions of the Burlington limestone and the lowest part of the Keokuk ; it has not been found 

 outside the vicinity of Burlington, Iowa, where it is one of the rarest fossils. Only two 

 species have been certainly recognized, as follows : 



The Species of Nipterocrinus 

 I. Arms relatively long. 



IIBr 6 to 9. IIIBr 10 to 15 or more N. wachsmuthi. 



II. Arms relatively short and strong. 



IIBr 4 or 5. IIIBr 6 or 7 N. arboreus. 



