188 



PALAEONTOLOGY OF ILLINOIS. 



Genus STROTOCRINUS, M. and W. 



((TzpioToq, spread j xptvov, a lily; in allusion to the spreading summit.) 



Synon. — Actinocrinus (sp.), Hall ; Meek and Worthen, and others; (not Miller.) 



Calathocrinus, Hall (subgen. Actinocr.), Feb., 1861. Descr. Crinoid. Prelim. 

 Notice, p. 12 ; (not von Meyer, 1848, Leouhard and Bronn's Jahrb., p. 469.) 



Generic Formula. — 



Basal plates, 3. 



Subradials, 0. 



Primary radials, 3+5. 



Secondary ditto, 1 to 2-j-lO, and succeeded by tertiary and other divisions, 

 which, with the interaxillary, interbrachial and vault pieces, unite to form 

 a greatly expanded, ten rayed, horizontal disc, completely isolating the anal 

 and interradial areas from the vault, and supporting the free ascending 

 arms around its margin. 



Interradial pieces, 9 to 10, or .more. 



Anal pieces, 9 to 13, or more ; the first one resting upon the basals. 



Arms, 30 to 72, or more. 



Fig. 5. 



Strotoerinus perumbrosus f 



This figure shows the expanded summit, natural size, as seen from below, 

 with the body broken away — some portions of the outer margin of the disc be- 

 ing restored. The radiating bars seen within the central cavity are the parti, 

 tions between the canals leading out to the rays. These canals are variously 

 divided and subdivided in the expanded disc, before reaching the arm bases 

 around the margins. The very small oral opening of the summit is seen a 

 little below the middle. 



