EHODOCRINIDJE. 219 



brachials^, the second row consisting either of two or three pieces. When 

 there are two plates at the regular sides, the anal side has always three, but 

 when the former has three, there is no additional plate at the anal side. 

 Ventral disk narrow, rising but little above the dorsal cup ; composed of 

 irregularly arranged plates, none of which can be recognized as orals. Disk 

 ambulacra subtegminal. Anus excentric, sometimes marginal. Column 

 round; the axial canal pentagonal or stelliform; the internodes frequently 

 consisting of but one joint. 



Distribution. — Khodocrinus first appears in America in the Hamilton 

 group ', it attains its climax in the Kinderhook and Burlington beds, and 

 becomes extinct at the end of the Keokuk epoch. Specimens, as a rule, are 

 rare. In Europe the genus occurs in the Rheinisch Uebergangsgebirge, and it 

 is represented by several species in the Mountain limestone. 



Ty^oe of the genus : Rhodocrinus verus Miller, from the Carboniferous of 

 England. 



Remarks. — Rhodocrinus verus, according to J. S. Miller, occurs in the 

 Mountain limestone of Yorkshire, and also in the Wenlock limestone of 

 Dudley, England, and it was said to have three basal plates. Miller con- 

 founded two very different types, which have since been recognized as 

 distinct genera. The Carboniferous form, which took Miller's specific name, 

 is universally regarded as the type of the genus Rhodocrinus, having frve 

 infrabasals instead of three, and biserial arms ; the Silurian form from Dud- 

 ley, with three infrabasals and single arm joints, was described by Phillips as 

 Sagenocrinus expansus. 



The genus Rhodocrinus, as we understand it, includes species with smooth, 

 nodose, and spiniferous plates. For a certain species with spinous plates 

 from the Devonian, Roemer proposed the genus Acanthocrinus. We have 

 carefully examined the figures of A. longispinus, as given by Wirtgen and 

 Zeiler,* but have failed to discover any characters by which this form can 

 be separated even subgenerically. Spinous projections on the basals and 

 radials occur quite frequently also among Carboniferous species in all possible 

 variations. It is possible that '^ Rhodocrinus gonatodes " Mliller belongs to 

 Oehlert's new genus Diamenocrinus . 



We have referred Rhodocrinus microbasilis and R. pyriformis, both of 

 Billings, to Archwocriniis ; R. vesper alis White to Diabolocrinus ; R. melissa 

 Hall to Lyriocrinus ; R. Halli Lyon to Thysanocrinus ; R. stellaris de Koninck 



* Verh. d. Naturbist. Verein, Jahrg. XII., Tab. II. 



