ICHTHYOCRINIDAE 325 



Metichthyocrinus tiaraeforrnis (Troost) Hall 

 Plate XXXIX, figs, ip-21 



Cyathocrimis tiariformis Troost, Araer. Jour. Sci., (2) VIII, 1849, p. 420, n. n. ; Miller, N. A. Geol. and 

 Palaeontology, 1889, p. 256; Troost (tiaraeforrnis) Proc. Amer. Assoc. Adv. Sci., 1850, p. 61, n. n. 



Ichthyocrinus tiaraeforrnis (Troost) in Hall, Geology Iowa, I, pt. 2, 1858, p. 558, text-fig. 74. — Shumard, 

 Trans. Acad. Sci. St. Louis, II, 1866, p. 378. — Wachsmuth and Springer, Revision Palaeocrinoidea, 

 pt. 1, 1879, p. 35. — Wood, Bull. 64, U. S. Nat. Museum, 1909, p. 99, pi. 7, figs. 15-17. 



Metichthyo.crinus tiaraeforrnis, Springer, Jour. Geology, XIV, 1906, p. 477. 



A large species. Crown globose, wider than high, very broadly truncate at 

 the base; greatest width about IIIBn; height to width at IIBn, i to 2; at I Ax, 

 1 to 2-7', spread of calyx from column to IIBn, 1 to 3.4; to I Ax, 1 to 2.4; cross- 

 section at IIB1-3 circular, above obtusely decangular; side outline convex. Sur- 

 face smooth, plates strongly imbricated. Maximum crown, 30 mm. high by 

 32 mm. wide. 



IBB minute. BB small, usually wholly within the column facet, which also 

 involves part of RR. IBn and corresponding higher brachials about three times 

 wider than high. IIBr 4, exceptionally 3; IIIBr 14 to 16, no further bifurca- 

 tions visible. Brachials evenly curved below, becoming convex about the second 

 bifurcation. Arms broadly rounded, closely abutting, and infolding. Column 

 facet indented, and very large. Column with short proximal columnals. 



The main character that distinguishes this species from M. clarkensis is its globose form, 

 with broad base, very low and broadly spreading calyx, and relatively shorter and wider 

 brachials. The two forms are found at the same horizon, though not certainly at the same 

 localities except at White's Creek. There seems to be a fair degree of constancy in the differ- 

 ences, slight though they are, and in a f orm with so few structural features apparent in the 

 fossil which can be used for specific determination we must take them for what they are 

 worth. From the type locality we have several good specimens, although the species is quite 

 rare. 



The first is the type, in the original collection of Troost now belonging to the U. S. 

 National Museum ; it is absolutely rotund and perfect in every particular, lacking, of course, 

 the stem (PI. XXXIX, figs, iga-c). It has three primary brachials above the radial in one 

 ray ; whether this is the right posterior ray cannot be ascertained, as the inf rabasals are con- 

 cealed by the top columnal ; but it will be seen from the figure that the radial in that ray is not 

 in any way dwarfed or different in shape from the others, as in Ichthyocrinus. I therefore 

 consider the extra plate in this ray as a mere case of sport, having no relation to a radianal. 

 I have two other good specimens with arms complete, neither of which show any additional 

 plate. The number of secundibrachs is constant at 4 except in one ramus of one ray in each 

 specimen. Besides these I have two other specimens from the same locality showing the base 

 and parts of the ray ; all five specimens have the same broad base, with basals wholly included 

 in the column facet. 



The question of the horizon of this species has been discussed under the genus. 



Type. In the United States National Museum, Troost collection. The other specimens 

 figured are in the author's collection. 



Horizon and locality. Lower Carboniferous, New Providence shales (Knobstone) ; 

 White's Creek Springs, Tennessee. 



