ICHTHYOCRINIDAE 285 



Ichthyocrinus corbis Winchell and Marcy 

 Plate XXXVI, figs. 1-7 



Ichthyocrinus corbis Winchell and Marcy, Mem. Boston Soc. Nat. Hist., I, 1865, pp. 89, 108. — Hall, 20th 

 Rep. New York St. Cab. Nat. Hist., 1867, pp. 384, 391. — Wachsmuth and Springer, Revision Palaeoc- 

 rinoidea, pt. 1, 1879, p. 34. — Miller, Jour. Cincinnati Soc. Nat. Hist., IV, 1881, p. 175, pi. 4, fig. 5; 

 N. A. Geol. and Palaeontology, 1889, p. 256, fig. 343. — Weller, Bull. 4, Chicago Acad. Sci., I, 1900, 

 p. 146. — Springer, Jour. Geology, XIV, 1906, p. 477. 



A large species. Crown ovoid, without prominent surface angularity; ex- 

 panding rapidly from base, with spread of calyx from RR to upper IIBr of 

 about 1 to 2.4; average height to width at this zone, about i to 1.6. Side outline 

 convex; cross-section circular. Surface smooth; plates of higher brachials 

 somewhat imbricated. Maximum incomplete crown to about fourth bifurca- 

 tion, 60 mm. high by 50 mm. wide; large specimen, 54 mm. high by 43 mm. wide 

 at upper IIBr; small specimen, 13 by 18 mm. 



IBB small, perhaps obsolete. BB small, probably not visible beyond the 

 column. Lower brachials evenly curved, rarely with obscure median ridge, or 

 becoming somewhat angular above. IIBr 4, IIIBr 6 to 8. Average height to 

 width of brachials above IBr, about 1 to 3. Complete crown unknown, but in 

 large specimens not infolding at the fourth bifurcation. Column unknown. 



This species is chiefly known from internal natural casts which have been collected some- 

 what abundantly in the magnesian limestone of Niagaran age- at Chicago. As usually found 

 the plates have been destroyed by chemical action, leaving a limestone cast of the interior 

 formed by the filling of the visceral cavity and replacement of the soft parts by the infil- 

 trating matrix. On these casts the suture lines at the junction of the plates in specimens 

 otherwise well preserved are sharply marked by thin, knife-like ridges. Even when not 

 destroyed the test is softer than the matrix and. rapidly disintegrates, leaving the impression 

 of the interior as before. But few specimens have ever been found with any portion of the 

 test preserved, and none with the calyx complete. 



It is a remarkable fact that almost without exception these specimens are rotund, being 

 but little flattened as is frequently the case with other species. This indicates that they were 

 imbedded in a very soft, homogeneous and plastic matrix, which surrounded and supported 

 them uniformly on all sides, replacing the soft parts before the pliant calyx became subjected 

 to any pressure except from the gravity of its own viscera. We find, however, in by far the 

 greater number of the specimens a peculiar distortion at the lower part of the calyx, where 

 to the height of about the first IIBr it appears as if bent in on one side while the opposite is 

 more than correspondingly ventricose, and the apex of the base points away from the vertical 

 axis of the crown (PI. XXXVI, fig. 3a). Among the few specimens found with any part of 

 the plates in situ none have the base intact, so that we cannot be certain of the exterior form, 

 although I have no doubt that it partook of the same distortion. The best one of these 

 (figs. 6a, b) shows that while the calyx wall is quite thick and strongly arched toward the 

 higher brachials, it becomes very thin in the lower part where the unsymmetric bending of 

 the wall occurs. 



When I first discovered the existence of the radianal on the convex side of a number of 

 specimens like figures 2, 3, I thought the asymmetry might be due to the anal structures ; but 

 other specimens like 4, 5, with the radianal on the concave side, disposed of this theory, 

 and I found that the distortion bore no fixed relation to the posterior side. It is not due to 



