Grabau — Biserial Ann in- Certain Crinoids. 297 



enlai'gecl so as to occupy the place of two plates. The last 

 three plates of the arm are rectangular. 



A spechnen of P. hemisphericus in the collection of the 

 National Museum (cat. 24,183) shows some very interesting 

 abnormal features. Fig. 8 represents two out of a group of six 

 arms. In the right hand member represented, uniserial con- 

 ditions obtain to the 18th plate. While most of these plates 

 are liiore or less cuneate, some rectangular ones are inter- 

 spersed. These apparently retain their primitive condition. 

 Biseriality begins abruptly with the 19th plate. The 22d 

 plate is a double one, or, at least, occupies the place of two. 

 It has either retained its primitive character or developed at 

 the expense of another plate, which became abortive. The 

 same thing holds true of plates 28 and 29. These seem to be 

 prim-itive plates. Plate 35 is developed at the expense of 

 2)late 36. JS'umber IT is again a uniserial plate. The terminal 

 plates are not well enough preserved to determine their char- 

 acter. The next adjoining arm has seven uniserial plates ; 

 i. e., to the 12th plate of the arm. This and the two preced- 

 ing ones are regularly cuneate. Above this, biseriality com- 

 mences abruptly and continues uniformly through plate 57. 

 Plates 58 and 60 appear abruptly as uniserial plates with plate 

 59 wedged in between them. Above plate 61 the arm divides 

 into two branches, the left branch continuing normally biserial, 

 the right one beginning with a uniserial plate. The terminal 

 portions of these branches are not preserved. The same speci- 

 men shows another bifurcating arm, and several more arms 

 with uniserial plates interspersed. Perfect specimens of this 

 species always show the uniserial terminal plates, so far as I 

 know ; this species being, next to Encrinus liliiformis, one of 

 the best to show this feature. 



Dlchocriniis inornatus W. and S. 



This species from the Kinderhook of Marshall Co., Iowa 

 (J^ational Museum collection), has three uniserial plates at the 

 base of the arm above the first division. After that the plates 

 become biserial. In the arm represented in ^^. 9 the 84:th 

 plate from the base has become biserial, while from the 85th 

 upwards uniserial conditions still persist. In this arm, the 

 apex of which is broken away, nine uniserial plates persist, the 

 topmost one having passed but little beyond rectangular form. 

 In a more perfect arm of the same specimen (fig. ] 0) twenty uni- 

 serial plates occur at the apex, the lower of which are cuneate, 

 while the upper closely approach rectangularity. 



In Wachsmuth and Springer's figure of the type specimen 

 of Dichocrinus hamiltonensis Worthen"^ the apical plates of 

 *Loc. cit., pi. 76, fig. 10. 



