INVERTEBRATES. 265 



supporting on their concave upper sloping sides the first divi- 

 sions of the arms. (Anal and interradial pieces unknown.) 



Arms, after the first division on the second radial plate, 

 bifurcating again on the second piece, and in at least two of 

 the rays, the inner branch bifurcates again once or twice on the 

 second piece. Above the last divisions the arms are slender, 

 nearly cylindrical, and consist of a single series of pieces for a 

 short distance, then gradually pass into a double alternating 

 series of small pieces. Tentacles rather thick where they 

 connect with the arms, but soon tapering, and apparently com- 

 posed of very long joints. Height of body, to summit of first 

 radials, 0.35 inch; breadth at the top of the first radials, 0.45 

 inch; breadth of base, 0.33 inch; height of ditto, 0.13 inch. 



This species appears to be rather closely allied to P. Georgii of Hall (Sup. 

 Iowa Report, p. 82), but has a smaller body in proportion to the size of its arms. 

 Its first radials also differ in being narrower above, in proportion to their 

 length, and have a comparatively smaller sinus in the upper margin. Again, 

 its second radials, and the two succeeding joints of each ray resting upon the 

 superior sloping sides of these, are proportionally larger. In consequence of 

 the smaller size of these pieces in P. Georgii, its second radials and the next 

 range of pieces above, are received within the sinus in the first radials, while in 

 P. Prattenanus the second radials alone fill these depressions. In both of these 

 species the column is somewhat compressed and more or less tortuous, but it is 

 much stronger in P. Georgii, and composed of comparatively thinner pieces. 



In comparing our species with Prof. Hall's description, it is necessary, in 

 order to have a clear understanding of the relations of these forms, that the 

 reader should bear in mind the fact that he has inadvertently, or by a typo- 

 graphical error, described the first radial plates of P. Georgii as the basal 

 pieces, and the second radials as the first brachials. 



Locality and position: Randolph county, Illinois; St. Louis division of the 

 Lower Carboniferous series. 



34 Sept. 13, 186G. 



