11 



beyond which they are simple to their extremities. The anterior 

 brachial is quadrangular, constricted above, and supports a single 

 arm, making 17 arms altogether for this species. 



The anal side of the specimen is distorted so that the number and 

 form of the anal plates cannot be clearly determined ; but three can 

 be seen ; the second one much larger than the others, and apparently 

 arranged as usual in this genus. 



Column moderately stout, round and composed at the summit of 

 evenly-sized plates, that alternate with thinner ones below. 



This species is nearly related to Pot. (Scaph.) unicus, of Hall, but 

 differs essentially from that in the number and bifurcation of the 

 arms. 



Position and locality: Upper beds of the Keokuk limestone, 

 Hamilton, 111. 



No. 170 of Mr. L. A. Cox's collection. 



PeTEEIOCKINUS ASPEB. N. SP. 



Body of medium size, basin shaped, base slightly depressed, the 

 basal plates being concealed by the columnar joints. Subradials 

 prominent, length and breadth about equal, the lower angles form- 

 ing a part of the basal concavity, and the upper angles depressed to 

 correspond with similar depressions in the lower margins of the 

 radial plates. 



Radials short, about as wide as long, pentagonal, depressed at 

 their lower angles, with a gaping suture between them and the suc- 

 ceeding brachials. 



Brachials two, the first quadrangular, the second sharply angu- 

 lar above, and but little narrower than the radial series. A slight 

 angular prominence extends lengthwise across the brachial plates, 

 and the same may be seen on the lower portion of the arms under 

 a good glass. 



Arms on the anterior ray, after the first division on the second 

 brachial, divide again on the fourth and sixth plate, and both the 

 outer divisions again on the eighth plate, while the inner one appears 

 to be simple after the second division, thus giving six arms to this 

 ray. The antero-lateral rays appear to divide once more, giving eight 

 arms to these rays. 



Arm pieces rounded, moderately wedge-shaped, and diminish 

 rather rapidly in width towards their extremities. Delicate pinnules 

 are given off from their longest sides. Anal plates and column un- 

 known. 



The depressions at the angles of the body plates, give a rugged 

 appearance to the body of this species, and will serve to distin- 

 guish it from the other forms with which it is associated. 



Position and locality : Upper part of the geodiferous shales of the 

 Keokuk group, one mile below Keokuk. 



No. 191, Mr. L. A. Cox's collection. 



