134 FOSSIL CRINOIDS. 



Wood's " Tripleurocrinus," I first thought they might go together; but the 

 type is so diiTerent that this seems impossible, and, as ah-eady shown, too much 

 importance must not be given to the form of the canal. 



Type. Author's collection. 



Horizon and Locality. Onondaga Group, Le Roy, Livingston county, New York. 



POTERIOCRINIDAE. 



The name Poteriocrinus has been applied among authors to a very large 

 number of species, whereas the fact is that, as genera are now understood, it 

 belongs to but few. As was the case with most of the genera established by 

 J. S. Miller in 1821, it came later on to be taken as the type of a family, and was 

 numerously subdivided as research became more exact and material more 

 plentiful. This was done especially by Wachsmuth and Springer in the Revision 

 of the Palaeocrinoidea ; and the genera then proposed by them, or adopted from 

 others, to be carved out of the parent genus, have for the most part been accepted 

 by subsequent authors as well founded and judicious. The number of recog- 

 nizable species in Poteriocrinus proper was reduced to thirteen, from which 

 probably two, hitherto unfigured, P. obuncus White, and P. whitei Hall, should 

 be removed; whi\e another Cyathocrinus macro pleurus Hall, in every way char- 

 acteristic, from the Lower Burlington Limestone, should be added. The last 

 named species, and another also very characteristic and beautiful species from 

 the Upper Burlington Limestone, P. doris Hall, have never been figured. The 

 possession of some excellent specimens for their illustration, and of specimens 

 of another very remarkable species from the Keokuk Group, herein described, 

 has suggested a brief discussion of the genus and its relations. 



It has been said with reason that Poteriocrinus is not typical of the family 

 group, Poteriocrinidae, to which its name has been applied by Wachsmuth and 

 Springer. The two leading characters which distinguish this family from the 

 other great family of dicyclic Inadunata, the Cyathocrinidae, are: — 



1. Pinnulate arms, as against non-pinnulate. 



2. A straight and usually wide radial facet, with transverse fulcral ridge, 

 as against a curved and usually narrow facet. 



It has been thought that Poteriocrinus, along with character No. 1, possessed 

 the radial facet of the Cyathocrinidae. This is only true in part ; it is really, as 

 to this character, an intermediate form, its radial facet being small in size, occupy- 

 ing only a part of the distal face of the radial, and therefore more or less round. 



