292 



Grahau — Biserial Arm in Certain Grinoids. 



the only perfect wedge-shaped plate, the succeeding ones 

 approaching a rectangular outline, which is nearly, if not quite, 

 reached in plate 85 by a rather abrupt transition. It will be 

 seen that this arm is retarded with reference to the adjoining 

 one (fig. la). Thus in arm h plate 81 is still wedge-shaped, 

 w^liile in arm a the corresponding plate has already become 

 biserial. Again, plate 85 in arm h is rectangular, or nearly so, 

 while in arm a this plate has already become perfectly cuneate. 



Fig. 2. Encrinus liliiformis. Diagram of arm showing uniserial apical 

 plates. (Coll. Bost. Soc. Nat. Hist.) 



Fig. 3. Platycriniis huntsvillae. Diagram of arm of young individual, 

 showing uniserial character of plates. Length of arm, 9"^'". (Kirk Coll.) 



Fig. 4. Platycrinus huntsvillae. Diagram of arm of youthful individual, 

 showing uniserial apical plates. (Kirk Coll.) 



Since we cannot postulate a difference in age between the two 

 arms of the same crinoid, we must consider that one is retarded 

 in development with reference to the other. 



