CONTRIBUTIONS TO PALiEONTOLOGY. 125 



three or more times, while in the 0. thiemei there are no true bifurcations 

 above the second one. The specimen is imperfect in its upper part. 



Geological formation and locality. In the shales of the Hamilton group, 

 associated with Platycrinus eriensi, Spirifer granuliferus, S. mucronatus, 

 Strophodonta demissa, Orthis penelope, and otlier characteristic Hamilton 

 fossils : Erie county, N.Y. C. A. White, collector. 



GENUS RHODOCRINUS (Miller). 



[SUBGENUS] ACANTHOCklNUS? ( R(emer). 



Admitting the formula of Rhodocrinus as now generally adopted 

 among authors, which recognizes five basal plates instead of three 

 as given by Miller, the genus will include several forms heretofore 

 referred to other genera. The formula given by De Koninck is as 



follows : 



Basal plates, 5; 



Subradials, 5; 



Radials, 3X5; 



Brachials, 1 or 2, soldered to or forming part of the calyx; 



Interradials, 6 to 8X4; 



Anals, 10 to 12; 



Interaxillaries, 1X5; 



Arms 10 to 20, bifurcating two or three times. 



Adopting this formula without other restrictions, we include 

 those forms with five rudimentary or undeveloped plates, as Thy- 

 sANocRiNus and Lyriocrinus of the Niagara group, as well as Olla- 

 CRiNus, GiLBERTSocRiNus aud AcANTHocRiNus, where the basal plates 

 are developed. 



In the simpler forms of Rhodocrinus, we have the three radial 

 plates, succeeded by one or two pairs of secondary radials or bra- 

 chial plates below the arm openings; with arms in pairs, with or 

 without bifurcations; and a simple interradial series of six or eight 

 plates, uniting with those of the rounded dome. 



These forms, which are usually globose or subglobose, begin their 

 existence in the Lower Silurian strata, and continue through inter- 

 mediate formations to the Carboniferous limestone. 



In Thysanocrinus we find a departure from those just noticed, 

 which include some of the typical forms of the genus. The described 

 forms of Thysanocrinus are not globose, but turbinate; the basal 

 plates rudimentary; the arms usually bifurcating from their origin; 

 the interradial plates usually three, with a single axillary or inter- 

 brachial plate. The character of the dome is not well determined : 



