RHODOCRINID^. 



217 



crinus. S. A. Miller in 1889 added Lyriocrinus^ and adopted Lyon's name 

 Goniasteroidocrinus in place of Ollacrinus. He amended this in the following 

 year by leaving out the monocyclic Iladrocrinus^ and adding Arcliceocrinus 

 and Baphanocrinus. 



To the genera which we arranged under this family in 1885,* w^e add the 

 genus Diabolocrinus, which we have proposed for a species that had been 

 previously referred by us to ArchcBocrinus. 



The Ehodocrinidae are nearest related to the Thysanocrinidae, but are 

 readily distinguished by the complete lateral separation of the radialsf by 

 the interradials ; the radials of the Thysanocrinidse being in lateral contact 

 except at the anal side. The marked asymmetry in the ventral disk, so 

 characteristic of the latter family, is not ob- 

 servable in the Rhodocrinidge, in wdiich the 

 whole calyx, as a general rule, is remarkably 

 symmetrical. 



The family has a great stratigraphic range, 

 extending from the Lower Silurian to the 

 middle of the Carboniferous and becoming 

 extinct in the Keokuk group. The ancestral 

 type is probably Arcliceocrinus in the Trenton 

 group, of which Diaholocrinus is an offshoot. 

 The evolution of these forms through Lyrio- 

 crinus in the Niagara, Tliylacocrinus and Eipi- 

 docrinus in the Devonian, to the profusely 



developed Rhodocrinus in the Carboniferous, is by easy gradations. Nor 

 is the step from Rlwdocrinus to the highly specialized Gilbertsocrinus a 

 difficult one, because there are transition forms in which the characters of 

 the two genera are to a great extent merged. Raphanocrimis in the Trenton, 

 and Anthemocrinus from the Upper Silurian of Gotland, apparently repre- 

 sent variations toward the Thysanocrinidse. 



The Ehodocrinidae are by flir the most important dicyclic family of the 

 Camerata, being composed of ten genera and fifty-four species, of which 

 thirty-six are from America, and eighteen from Europe. 



* Revision, Part II., pp. 96 to 99. (Proceed. Acad. Nat. Sci. Piiila., pp. 318-321.) 

 t There is an occasional exception to this in the genus Lyriocrirms, where the radials are sometimes 

 connected by a narrow strip, except at the anal side. This occurs quite frequently in L. dactylus from the 

 Niagara of New York; wliile in Z. melissa, L.juvenis, and an nndescribed Lyriocrinm from Dudley, England, 

 the radials are widely separated. 



28 



Tig. 10. — Rlwdocrinus. 



