842 PROCEEDINGS OF THE ACADEMY OF [1885. 



D. coxanus Worthen, 1882, Bull, i, Illinois State Mus., p. 35, and Geol. Rep. 111., 

 vol. vii, p. 313, PL 27, fig. 7. we take to be a mere synonym of Dichocrinus ficus. 

 1882. D. hamiltonensis Worthen, Bull, i, 111. State .Mus. Nat. Hist., p. 35; also Geol. 

 Rep. III., vol. v ii. p. 313, PI. 27, fig. 10.— Keokuk liuiest. Hamilton, 111. 



TALAROCRINUS W. & Sp., Rev. ii, p. 85. 

 Additional species : — 



1882. T. ovatus Worthen, Bull, i, Illinois State Mus. Nat. Hist., p. 36; also Geol. 

 Rep. 111., vii, p. 314, PI. 19, fig. 11.— Kaskaskia, gr. Monroe Co., III. 



PTEROTOCRINUS Lyon & Cass., Rev. ii, p. 87. 



Family VIII.— ACROCRINID^ W. and Sp. 



The Acrocrinidae, so far as known, are represented by a single 

 genus, and of this only three species have been described, two 

 from the Chester (Kaskaskia) limestone, and one from the coal 

 measures of America. 



No attempt has ever been made to assign the genus Acrocrinus 

 to its proper systematic position. Zittel and He Loriol in their 

 classifications omit it entirely, and the descriptions by Yandell 

 and llall, which were from imperfect specimens, are indistinct and 

 partly incorrect. Thanks to the kindness of Prof. Worthen, we 

 have been able to examine a very perfect specimen of an unde- 

 scribed species, which one of us described for volume vii of the 

 Illinois Geological Report, and of which preliminary descriptions 

 were published in Bulletin I, of the Illinois State Museum of Nat. 

 Hist., p. 41. The specimen shows plainly that the base is bipar- 

 tite, as Hall suspected, and not undivided, as stated by Yandell. 

 Fortunately the other plates of the calyx were also in place, and 

 in a condition to be critically examined. 



Acrocrinus departs from most Palaeocrinoidea in two important 

 particulars, and upon these, mainly, the present family is founded. 

 First : The plates of the calyx, which in all other species with 

 large numbers of plates decrease in size from the basals to the 

 arm bases, in Acrocrinus exhibit a decided increase in the same 

 direction. Second : The radials are not connected with the basals, 

 but separated from them by several rings of plates, which in po- 

 sition are partly radial, partly interradial, and which apparently 

 are not represented in other genera of the Palaeocrinoidea. 

 This peculiar structure renders it exceedingly difficult in this 



