Grabau—Biserial Arm, in Certain Crinoids. 299 



are in contact for a short distance. Normally, in the adult, 

 the plates of the arm above the middle are cnneate, but above 

 this they are again rectangular to the tip. This cuneate char- 

 acter of plates is much less marked and often wanting entirely 

 on young individuals. 



Eiicalyptocrinus o calls Hall. 



In a young specimen of this species from the J^iagara of 

 Waldron, Ind., the terminal plates, though not quite clearly 

 defined, appear to extend entirely across. They furthermore 

 seem to be cuneate to the tip. It thus seems that this species 

 has its terminal arm plates introduced in cuneate form. 



Hydreionocrinus dexjressus (Troost). 



This species from the Chester of Sloans Yalley, Ky. (Kirk 

 Coll.), is remarkable for the curious development of its arm 

 plates. (Fig. 11.) The first brachial is large, spine-bearing and 

 axillary. Upon this follows on each side a simple, nearly 

 rectangular plate, each followed in turn by one or two sets of 

 interlocking or biserial plates. This is again followed by a 

 single spinous axillary plate, which is in its turn succeeded on 

 each upper face by a single large plate. On the inner branches 

 a large number of biserial plates follow immediately upon this 

 plate, continuing to near the apices of the arms, where they 

 become uniserial, with rectangular terminal plates. The outer 

 arms branch again, only a few biserial plates intervening 

 between the first plate and the simple spinous axillary of the 

 new division. The inner one of tiie branches is again simple, 

 with biserial plates, uniserial at the aj)ex ; while the outer arm, 

 after a few biserial plates, branches again, making the fourth 

 division. Again the inner arm continues simple, with biserial 

 plates changing above into uniserial, with quadrangular ter- 

 minal plates. A final division, the fifth, occurs in the outer 

 arm after a biserial interval. Each of the final divisions is 

 biserial at first, but uniserial at the apex. It may be noted 

 that as we proceed upwards an increasing number of biserial 

 plates intervenes between the base of a branch and its division. 



We have now seen that in the representatives of the three 

 most important orders of crinoids, the Articulata, the Camerata, 

 and the Fistulata, the terminal arm plates are introduced as 

 uniserial quadrangular ossicles, which later change to cuneate 

 and finally to a biserial interlocking condition. It may there- 

 fore be considered as highly probable that all arm plates are 

 normally introduced as q uadrangular plates in a uniserial con- 

 dition. In primitive forms they never pass beyond that con- 

 dition. In moi'e specialized types the earliest arm plates alone 



