1879.] NATURAL SCIENCES OF PHILADELPHIA. 277 



twenty, perhaps more in some species; the first one large, resting 

 between the first and second radials; the succeeding ones smaller, 

 rapidly decreasing in size and thickness upward, and having an 

 inward curvature. They are followed by very minute, irregular 

 polygonal plates, which form the interradial portion of the vault. 

 The radial summit areas consist of two rows of somewhat larger 

 plates, alternately arranged, which extend to the ventral covering 

 of the free rays, and probably throughout their full length. In 

 the median portion of the vault, there are six rather thin but large 

 apical dome plates. The summit, when the rays are extended, is 

 not higher than the top of the second radials; general surface de- 

 pressed. Column heavy, composed of very thin joints, tapering 

 rapidly downward; central perforation above medium size, pen- 

 tagonal. 



Onychocrinus is most nearly related to Taonocrinus, with Avhich 

 it is almost identical in the construction of the anal portions. It 

 differs, however, materially in the free rays, the arm structure, and 

 its greater expansive power. 



Lyon and Casseday took the free rays to be arms, and the true 

 arms to be pinnulae. These authors did not extend the genus to 

 forms like their Forbesiocr. ramulosus, which has only four radials 

 instead of five, as stated in their generic formula, though that 

 species has all the other characters of the genus. 



Hall has never recognized Onychocrinus as a genus, but placed 

 0. asteriaeformis ^ a typical form, under Forbesiocrinus. Meek 

 and Worthen, evidently considering the small lateral proboscis 

 the principal distinction between Onychocrinus and Forbesiocri- 

 nus, and not knowing that all Taxocrini have it, placed all species 

 of this group in which they observed this appendage under Ony- 

 chocrinus. 



Geographical and Geological Distribution Onychocrinus has 



been found only in the Subcarboniferous of Ireland and the United 

 States ; it embraces the following species : 



1861. Onychocr. asteriseformis Hall. (Forbesiocr. asteriaeformis), Descr. New 

 Crin. Prelim. Not., p. 9; also Bost. Journ. Nat. Hist., vol. vii No. 2, p. 

 320. Meek and Worth Onychocrinus asteriaeformis Geol. Rep., Illinois, 

 vol. ii. p. 243. Upper Burlington limest. Burlington, Iowa. 



1866. Onychocr. diversus Meek and Worth. Proceed. Acad Nat. Sci Phila., p. 

 256 ; also Geol. Rep., Illinois, vol. iii. p. 492. Upper Burlington limest. 

 Burlington, Iowa. 



