330 PROCEEDINGS OP THE ACADEMY OP [1879. 



originating on the longer side of the plate; the line of articulation 

 between first and second radials more or less gaping exteriorly, 

 and the edge of the plate rounded. Among the species referred 

 to Scaphiocrinns there are some with two brachials (three radials, 

 Hall), others with but one; some with long slender brachials, 

 others with short ones; some in which the brachials occupy the 

 entire width of the upper margin of the radials, others scarcely 

 one-third, and with a horseshoe-like articulating scar; some with 

 a shallow almost concave calyx, others with a nearly cylindrical 

 one; in some species the arms are simple, with short joints, in others 

 branching, with long joints, or vice versa; in some the pinnulae 

 are strong, and the arms zigzag, in others thin and short, and the 

 arms straight; but all agree, with the exception of the few species 

 which we refer to Graphiocrinus (and among the latter Hall's 

 Scaphiocr. simplex), in the construction of the anal area, which is 

 exactly as described in Poteriocrinus, and in having a strong 

 cylindrical or slightly club-shaped ventral sac. 



Our own attempt to subdivide Poteriocrinxis subgenerically met 

 with but little better success, though we had the advantage of 

 subsequent discoveries. A division based upon the number of 

 free radials or brachials as they are now called cannot be carried 

 out practically. Species with two brachials are not otherwise 

 distinct from those with but one, and Hall himself, who founded 

 Scaphiocrinus upon the presence of a single brachial, and likewise, 

 Meek and Worthen, included therein several species with an addi- 

 tional brachial. In this they were evidently justified, for the 

 additional plate, according to our views, is only a supplementary 

 piece, which facilitates the motion of the arms, but has no effect 

 upon the general structure of the animal. The two brachials 

 combined have the form and size of the single one in fact it is a 

 compound plate, and this term properly expresses its relations. 



Nor can the gaping sutures be regarded as of generic import- 

 ance. We look upon this structure merely as another mode of 

 articulation taking place upon a straight hinge line instead of a 

 sloping semicircular scar and the gradations from one form to 

 another are so close, that it is impossible to separate groups thereby. 

 Nevertheless, to facilitate the identification of species, w r hich is 

 alwa3 r s difficult when there is a large number in a single group, 

 we have separated Poteriocrinus, as well as could be ascertained 

 from the descriptions and figures, with the aid of our own exten- 



