256 PROCEEDINGS OF THE ACADEMY OF [1879. 



1. ICHTHYOCRINUS Conrad. 



1836. Euryocrinus. Pliill. Geology of Yorksh., p. 205. 



1842. IcMliyocrinus, Conrad. Journ. Acad. Nat. Sci. Philad., vol. 



viii. p. 279. 

 1851. IcMliyocrinus, d'Orbigny. Conrs. elem. Pal. ii. p. 144. 

 1851. IcMliyocrinus, Hall. Geol. Rep. N. York, vol. ii. p. 195. 

 1878. IcMliyocrinus, Angelin. Iconogr. Crinoid., p. 13. 

 1878. IcMliyocrinus, Wachs. & Springer. Proceed. Acad. Nat. Sci. 



Philad., p. 252. 



A. Typical form. 



General form of body, including arms, ovoid to pyriform, 

 with almost equilateral pentamerous symmetry. Calyx bowl or 

 cup-shaped. Underbasals three, rudimentary, of unequal size, 

 sometimes seen only within the calyx. Basals five, very small, 

 their upper angles acute. Radial plates of adjacent rays alter- 

 nately arranged. Primary radials three to four by five, wide and 

 short, height about equal, but increasing in width rapidly upward, 

 the plates being wider at their upper margins than .at the lower. 

 Secondary and tertiary radials similar in form to the primaries, 

 quadrangular in general outline, though realty pentangular and 

 hexangular, those of the same order of equal height, and half the 

 width of those of the next order. Arms twenty to sixty or 

 more, accumbent, infolding at the tips, and forming with the calyx 

 an apparently solid structure. They are composed of single joints 

 which are heavy, wider than high, quadrangular, usually with wav- 

 ing sutures. Arm furrows shallow, tripartite. Pinnulre unknown. 

 Interradial and anal plates generally absent or undeveloped at 

 the outside, I. nobilis, Wachs. & Spr. alone, to our knowledge, 

 possessing both; the}^ are longitudinally arranged, but the anal 

 side cannot be distinguished. Vault unknown. Column com- 

 posed of very short joints near the top, increasing gradually in 

 length downwards. Central perforation of medium size, pen- 

 tagonal. 



The most striking feature of this genus, by which it is easily 

 recognized, is its symmetrical, equilateral figure, and this pervades 

 the whole body. 



Hall, in his diagram (Geol. Rep., N. York, vol. ii., pi. 45, fig. 2), 

 figures a small intercalated plate in line with the subradials, which 

 is evidently accidental. He does not mention it in his generic 

 description, nor can we find it in any of our specimens. Angelin 



