384 PROCEEDINGS OF THE ACADEMY OP [1881. 



founded two very different species, which have since been 

 recognized as distinct genera. The Silurian form, with three 

 underbasals and single arm joints, is now known as Sagenocr- 

 expansus ; the Carboniferous form, which took Miller's specific 

 name, is universally regarded as the type of Ehodocrinns^ having 

 five underbasals instead of three, and the arms composed of two 

 rows of interlocking plates. 



Before the true number of underbasals in Rhodocrinus was 

 ascertained, Phillips, in 1836 proposed for some allied species, in 

 which he discovei'ed five proximal plates, the genus Gilhertso- 

 crinus, but these species for other reasons have since been 

 referred to Ollacrinus, which was proposed by Cumberland in 

 1826. Bhodocr. crenatus Goldfuss has been referred to Eipido- 

 crinus^ and Bhodocr. quinquelobus Schultze to Eucriniis. 



The genus Bhodocrinus, as now restricted, is decidedl}' Sub- 

 carboniferous. The few Devonian species which we are unable 

 to separate from it bear more or less resemblance to Acantho- 

 crinus Roemer, and have been partly referred to that genus by 

 other writers. We have examined with great care the figures of 

 Acanthocr. longispinus from Niederlahnstein as given by Wirtgen 

 and Zeiler, and some excellent specimens of our own, but 

 have failed to discover any characters ])y which this form 

 may be separated even subgenerically. The spinous projections 

 on the apical plates (basals and first radials) are also found in 

 some of the Subcarboniferous species in all degrees of variation, 

 and are more or less the rule in Ollacrinus. It is even not 

 improbable that Roemer's imperfect type specimen A. longispinus, 

 which was said to be from the Subcarboniferous, is really an 

 Ollacrinus, and generically distinct from the Devonian specimens 

 which Wirtgen and Zeiler identified with that species. 



The two Austins placed Rhodocrinus under the Actinocrinidae, 

 D'Orbigny and Pictet under the Cyathocrinidae, Roemer and 

 Zittel made it the type of a family. 



Generic Diagnosis. — Body subglobose to semi-ovate, often 

 wider than high, with a concavity at the base, which frequentl}^ 

 includes not only basals but partly the first radial plates. Calyx 

 constricted toward the arm bases, its symmetry almost perfectly 

 equilateral ; surface of plates from entirely smooth to convex 

 and nodose, or striated with regular nodes or spines on basal, 

 first radial, and first interradial plates. 



