B. Descriptive. 

 RETEOCRINID^ W. and Sp. (1885). 



The lower plates op the rays more or less completely separated from those 

 of other rays, and from the prlmary interradials, by irregular supple- 

 MENTARY PIECES. Anal interradius divided by a vertical row of conspicu- 

 pous plates. (Dicyclic or Monocyclic.) 



Analysis of the Genera. 



Arms uniserial. 



a. Raclials separated all around. 



Dicyclic. Infrabasals 5. Column pentangular Eeteocrinus. 



Monocyclic. Basals 4. Column square . Xenocrinus. 



1). Radials separated only at anal side. 



Monocyclic. Basals 5. Column obscurely pentangular . . . . . Tanaocrinus. 



Geological and Geographical Distribution. 

 Number of known species. Only known in America. 



Formation. 



Eeteocrinid^. 



Americau. 



Reteocrinus. 



Xenocrinus. 



Tanaocrinus. 



Hudson Eiver. 



3 



2 



1 



Trenton. 



1 



- 



- 



Total species ... 7 



4 



2 



1 



Remarks. — This family was established to receive a little group of Lower 

 Silurian Crinoids, distinguished from all other knowm forms by the irregular- 

 ity and absence of definite arrangement in the interradial plates, caused by 

 the intercalation of minute supplementary pieces. These small pieces were 



