294 



Grahau — Biserial Ann in Certain Crinoids. 



In this series, specimen 4 is the oldest, all the plates up to 

 the 89th having matured or nearly so, while specimen 5 is the 

 youngest, only forty-two plates having matured. 



Quenstedt (Petrefactenkunde Deutschlands, iv, p. 461, pi. 

 106) figures what he considers the apex of a perfect arm. His 

 figure (164) shows biseriality to the end, but with the plates 

 more Avedge-shaped. He figures a very young individual of 

 Encrimis liliiformis (fig. 178, p. 406) in which the arms are 



100 



Fig. 5. Platycriuus liuntsvillae. Diagram of basal plates of adult arm. 

 (Kirk Coll.) 



Fig. 6. Platycrinns hemisphericus. Diagram of arm showing terminal 

 uniserial plates, (Harv. Univ. Pal. Lab. Coll.) 



uniserial throughout, the plates of the middle arm, however, 

 showing a wedge character. The terminal ones are perfectly 

 quadrangular. The same thing is true, according to Beyrich, 

 of the young of Encrinus gracilis. 



Quenstedt also figures several cases of abnormal develop- 

 ment of the arm tips, in which the suddenly constricted apices 

 are uniserial. (Loc. cit., figs. 175 and 181.) 



