322 SMITHSONIAN INSTITUTION 



Metichthyocrinus burlingtonensis (Hall) 

 Plate XXXIX, figs. 1-7 



Ichthyocrinus burlingtonensis Hall, Geology, Iowa, I, pt. 2, 1858, p. 557, text-fig. 75. — Quenstedt, Petref. 

 Deutschlands, IV, 1876, p. 515, pi. 108, fig. 24. — Wachsmuth and Springer, Revision Palaeocrinoidea, 

 pt. 1, 1879, p. 34. — Whitfield, Mem. Amer. Mus. Nat. Hist., I, 1893, p. 35, pi. 3, fig. 24. — Keyes, Geol. 

 Surv. Missouri, IV, 1894, p. 223. 



Metichthyocrinus burlingtonensis, Springer, Jour. Geology, XIV, 1906, pi. 6, fig. 15. 



A small species. Crown ovoid, higher than wide, greatest width about 

 IIIBr=; height to width at IIBr 3 , 1 to 1.6; at I Ax, 1 to 2.4; spread of calyx from 

 column to IIBr 3 , 1 to 4; cross-section circular; side outline convex. Surface 

 smooth. Maximum crown, 22 mm. high by 19 mm. wide; average of 5 speci- 

 mens, 18 mm. high by 17 mm. wide. 



IBB small, limited to the column facet. BB small, usually visible as small 

 triangles beyond the column. RR but little involved in column facet. IBn and 

 corresponding higher brachials about 1 to 2.4 in height to width. IIBr usually 

 3, sometimes 4. IIIBr 10 to 15 or more. Rays evenly curved, without angular- 

 ity or median ridge ; interlocking to at least the upper IIBr, above that abutting, 

 infolding, and tapering rapidly from about the third IIIBr, apparently without 

 further bifurcation. Column facet large, indented. Column large, round, 

 slightly enlarged at the top; composed near the calyx of very short columnals 

 which change somewhat abruptly to ossicles of four times their length alternat- 

 ing with very short ones. 



This was the first Carboniferous Ichthyocrinus described, although in connection with it 

 Hall in a note gave a description and diagram of Troost's unpublished species, /. tiaraeformis. 

 He had only a fragmentary specimen, in which the rays were not preserved beyond the first 

 axillary plate. Neither did it show the infrabasals, which were not mentioned in the descrip- 

 tion nor in the generic formula, although in his redescription of /. laevis from the Silurian 

 Hall had called attention to them as " three small, undeveloped plates." As usually found, 

 the top columnal remains attached in the sharply indented column facet and conceals these 

 plates from view; in one specimen (PL XXXIX, fig. 5&) they are of fair size externally, but 

 the interior view in another specimen a little larger than this (6b) shows how they diminish 

 in size at the inner side of the calyx. 



In another interesting specimen (fig. 7) we have an actual vertical section of the crown 

 through the axial canal, showing the calyx wall and the relative positions of infrabasals and 

 basals ; it also shows the probable position of the tegmen, though not its structure. The 

 transverse line from the margin of the axillary primibrach represents the upper surface of a 

 very definite granular structure, entirely replaced by crystallized calcite, but wholly distinct 

 from the objects above it which are a mixture of matrix and displaced ends of arms; the 

 integument is doubtless somewhat sunken from its original level, which may have been at the 

 height of the third secundibrach. 



The shape of the crown in this species is an elongate ovoid with the widest end up. In 

 this respect it is similar to M. clarkensis, but is well distinguished from M. tiaraeformis. 

 The same thing may be said as to most of the other characters, except the number of secundi- 

 brachs, which in this species is generally 3, and in the others 4. In five specimens of 

 M. burlingtonensis preserving the rays these plates appear as follows : Out of 43 rami in 



