298 



Grcibau — Biserial Arjii in Certain Orinoids. 



the arm are re})reseiited as imiserial. As nearly as can be 

 determined from the figure, there are fourteen uniserial plates 

 above the biserial ones. The lower are cuneate, the uppermost 

 quadrangular. In the text the arms are stated to be " composed 

 of long, cuneate plates, which slightly interlock." I have not 

 seen this specimen, but it is highly probable that Mr. Wester- 

 gren's figure of it represents the true characters. 



Acrocrinus amphora W. and S. 



A specimen of this rare species from the St. Louis of Hunts- 

 ville, Alabama, in the collection of Mr. E. G-. Kirk, has an 

 unusually well preserved arm. Eighteen or nineteen of the 

 terminal plates of this arm are uniserial ; the apical one is 



Fig, 9. Dichocriniis inornatus. Diagram of arm showing terminal uni- 

 serial plates. (U. S. Nat. Mus. Coll.) 



Fig. 10. Dichocrinus inornatus. Diagram of apex of arm showing numer- 

 ous uniserial plates. (U. S. Nat. Mus. Coll.) 



Fig. 11, Hydreionocrinus depressus. Diagram of part of an arm group, 

 showing mode of branching, and terminal uniserial plates. (Kirk Coll.) 



quadrangular, the one next below nearly so. The third and 

 fourth from the tip are slightly truncate, the fifth to eighth 

 cuneate, and the ninth to eighteenth cuneate with the point of 

 the wedge extremely thin and slender, but extending entirely 

 across. The nineteenth to twenty-first from the tip mark the 

 transition from the uniserial to the bisei'ial. 



Zeacrhms commaticus Miller. 



This species is normally uniserial throughout. Specimens 

 from the Warsaw limestone of Boonville, Mo., show an approach 

 to biseriality in a few of the adult plates, which do not extend 

 entirely across, so that the two cuneate plates of the same side 



