B. Descriptive. 
RETEOCRINIDAD W. and Sp. (1885). 
THE LOWER PLATES OF THE RAYS MORE OR LESS COMPLETELY SEPARATED FROM THOSE 
OF OTHER RAYS, AND FROM THE PRIMARY INTERRADIALS, BY IRREGULAR SUPPLE- 
MENTARY PIECES. ANAL INTERRADIUS DIVIDED BY A VERTICAL ROW OF CONSPICU- 
POUS PLATES. (Dicyciic or Monocyctic.) 
Analysis of the Genera. 
Arms uniserial. 
a. Radials separated all around. 
Dicyelic. Infrabasals 5. Column pentangular. . . . . . . . ReErroorinvs. 
Monocyclic. Basals4. Column square ...... . . . . XENOCRINUS. 
b. Radials separated only at anal side. 
Monocyelic. Basals 5. Column obscurely pentangular . . . . . TANAOCRINUS. 
Geological and Geographical Distribution. 
Number of known species. Only known in America. 
Formation. ’ RETEOCRINID&. 
| American. Reteocrinus. Xenocrinus. Tanaocrinus. 
Hudson River. 3 2 1 
Trenton. 1 7 2 
Total species . . . 7 4 2 1 
Remarks. — This family was established to receive a little group of Lower 
Silurian Crinoids, distinguished from all other known 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 
