296 



Grdbobu — BiseriaT Arm in Certain Crinoids. 



of this arm uiiiserial ; i. e., up to and including the 18th arm 

 plate. After this, biserial conditions prevail to the 103d plate, 

 which is cuneate and uniserial. Tlien the plates become more 

 and more truncated, until the 110th plate is nearl}^ rectangular. 

 In this species, according to Wachsmuth and Springer, biseri- 

 ality appears late in life. They cite a specimen in which the 

 crown measures 22™'" while the arms are still uniserial to the 

 tips.* 



Fia. 7. Platycrinus hemisphericns. 

 character of plates, (Harv. Univ. Pal 



Fig. b. Platycrinus hemispliericns. 

 irregularity in plates, and a bifurcation 



Diagram of part of group showing 

 Lab. Coll.) 



Diagram of two arms showing great 

 . (U. S. Nat.-Mus. Coll.) 



Fig. 



represents the outer arm of a group of six in another 

 specimen. The transition plates from uniserial to biserial at 

 the base are broken away, but there are at least eight or nine 

 of tlie unisqrial ones. After that (fi'om about the thirteenth 

 plate from the base of the group) biserial conditions obtain 

 through the llTth plate, after whicli the plates are again 

 uniserial. An irregularity occurs in plate 119, whicli is 



Loc. cit.. 



704. 



