2G6 PALAEONTOLOGY OF ILLINOIS. 



Platycrinus penicillus, M. and W. 



PI. 19, Fig. 6a, 66, 6 e. 



riatycrinus penicillus, Meek and Worthen, Sept., 1860. Proceedings Acad. Nat. Sci., 

 Philad., p. 380. 



Body small, cup-shaped below the arms, a little wider than 

 long, composed of thin plates, connected by moderately distinct 

 sutures, and ornamented by rather distant granules, which 

 show a tendency to range in radiating rows on the first radial 

 plates. Base depressed basin-shaped, considerably wider than 

 high, and broadly truncated, with a small marginal rim below; 

 plates firmly anchylosed. First radial pieces nearly flat, and 

 a little higher than wide, widening somewhat from the base, 

 subquadrangular, the upper angles being slightly truncated by 

 the anal and interradial pieces ; sinus in the upper margin, for 

 the reception of the second radial pieces, shallow, and appa- 

 rently about half as wide as the superior edge. Second radials 

 small, triangular, wider than long, and supporting on their 

 upper concave sloping edges the first divisions of the arms. 

 Anal and interradial pieces unknown. 



Arms above the first bifurcation of the second radial pieces, 

 with the two principal divisions subdivided on the second 

 piece, and the two middle branches each bifurcating again on 

 the second piece, beyond which they appear to be all simple, in 

 the only specimen yet obtained. Immediately above the last 

 bifurcations, the arms all consist of single series of pieces, 

 which are alternately longer and shorter on opposite sides; 

 these pieces become shorter and more wedge-shaped above, and 

 gradually pass into a double series of small alternating pieces. 

 The larger single arm-pieces below the bifurcations, and for a 

 short distance above, are generally longer than wide, narrow 

 around the middle, and expanded at each end, the superior 

 lateral angles projecting for the reception of tentacles. All 

 the tentacles are made up of joints two or three times as long 



