Grabaii — Biserial Arm in Certain Crinoids. 295 



Platycrinus. 



Wachsmuth and Springer lig'ure a young Platycrinus ameri- 

 camcs (N. A. Camer. Crinoidea, pi. 75. fig. 11) in which the 

 arm joints are long and slender, with a uniserial arrangement 

 throughout and wavy in outline. In adults of the same species 

 the arms are biserial, except for a few joints near the calyx, 

 the lowest of which are rectangular. Upward, these pass into 

 wedge-shaped plates by a gradual transition. But the transi- 

 tion from uniserial cuneate to interlocking biserial plates is 

 quite abrupt. 



Young specimens of Platycrinus huntsvillcB (Troost) from 

 the St. Louis group of Huntsville, Alabama, show uniseriality 

 throughout. The arm represented in fig. 3 is almost 9""^ 

 long. There are thirteen joints including the basal one, i. e., 

 the first brachial ; and all are more or less rectangular and 

 much longer than wide. The cuneate stage, through which 

 most of these joints will pass, is suggested by the manner in 

 which the plates bulge on the pinnulate side. The pinnules at 

 this stage are relatively large, having more tlie aspect of 

 branches. 



A somewhat older specimen is shown in fig. 4, the length of 

 the arm being 12°'"'. The 8th plate has become more wedge- 

 like than the preceding ones, and the 9th, 10th, and 11th are 

 I'egularly cuneate. Plates 12 to 21 represent the biserial stage, 

 the first and last of these being transitional. Plate 22, how- 

 ever, is a uniserial cuneate plate, and so are plates 23 and 24. 

 Plate 25 is truncated, while 27 is rectangular. It is thus seen 

 that the biserial plates do not appear as such after that stage 

 in development has been reached, as held by Wachsmuth and 

 Springer; but that the newly formed plates at the apex are 

 uniserial and rectangular, and gradually change througli cuneate 

 to a biserial interlocking condition. 



Tn tlie adult of this species (fig. 5) the biserial condition has 

 been pushed down to the 8th plate, while plates 7 and 8 have 

 become cuneate. But somewhat more retarded individuals 

 have been observed which, though of adult size, had eight and 

 even nine uniserial plates at the base of the arm. A single 

 specimen was found in which eleven uniserial plates occurred 

 at the base of the arm, though the upper two or three were 

 sti'ongly cuneate and of a transitional character. 



Two specimens of Platycrinus hemisjpliericus from the 

 Eeokuk of Crawfordsville have the arms shown in figures 

 6 and 7. In this species there are five groups of arms, each 

 consisting of six or eight arms. Fig. 6 represents the second 

 arm from the right, of a group of six, this arm becoming free 

 above the fifth brachial. There are about thirteen brachials 



