162 



PROCEEDINGS OF THE ACADEMY OP 



typical species of Actinocrinus. Hence, if we admit these latter groups, either 

 as genera or subgenera, consistency at least, if not indeed a philosophical system 

 of classification, would require that equal prominence should be given to these 

 corresponding sections of the Platycrinus group. Whatever theoretical views 

 may be entertained on this subject, however, the practical difficulty of ascer- 

 taining the nature of the summit, and the position of the aperture in palaeozoic 

 crinoids, will prevent the general distribution of the species into groups, upon 

 characters of this kind, unless a more profound study of great collections of 

 the remains of these animals, may yet bring to light some coincident, but 

 more easily observed characters, in the structure of the body, or other parts. 



Platycrinus Niotensis, M. and W. 



Body below the summit of the first radial pieces cup-sbaped, wider than 

 high ; sides slightly ventricose above the base. Base basin-shaped, several 

 times as wide as high, moderately expanding, rather broadly truncated below, 

 its lower margins projecting slightly downwards around the end of the column, 

 and provided with three very small projections, one at the lower extremity of 

 each of i's sutures. First radial plates large, higher than wide, widening 

 slightly from below upwards, subquadrangular or with the superior lateral 

 angles a- \ittle truncated by the interradial pieces ; each with a concave facet 

 for the reception of the second radials, equalling about half its breadth above, 

 and excavated near one-fourth its length on the outer side, below the upper 

 margin. Second radial pieces trigonal, very small, or scarcely filling the facet 

 or excavation in the upper side of the first radials ; rounded below, with each 

 superior sloping side supporting secondary radials, on the second of which 

 another bifurcation takes place, making four arms to each ray. 



Arms after the second division described above, simple, and at first composed 

 each of a single series of wedge-shaped plates, but soon passing into a double 

 series of small interlocking pieces, supporting on each side of the arms closely 

 arranged series of long-jointed tentacles. 



Column near the base compressed and tortuous, being composed of alter- 

 nately thicker and thinner elliptic pieces, with a very minute central per- 

 foration. 



Surface somewhat granulose ; sutures not grooved, nor distinctly apparent ; 

 those between the basal pieces indicated by a faint linear ridge. 



Height to summit of first radials, 0-30 inch ; breadth about 0-40 inch. 

 Grea'er diameter of column at base of body, 012 inch ; smaller do. 0-09. 

 Breadth of one of the arms, 006 inch; length of do. apparently an inch or 

 more. 



In its general appearance, this species is not unlike P. sarce of Hall, (Iowa 

 Report, p. 673, pi. 18, fig. 4), though it is much smaller, has a proportionally 

 much shorter base, and also differs in having but four, instead of six arms to 

 each ray. 



Locality and position. Niota, Hancock County, Illinois. Keokuk division 

 of Subcarboniferous Limestone. 



Platycrinus hemisph^ricus, M. and W. 



Body rather above medium size, hemispherical, being rounded below, and 

 about twice as wide as high ; base broad basin-shaped, and forming about 

 one third the entire height of the cup, with a pentagonal outline as seen from 

 below; columnar facet between one-third, and one-fourth the diameter of the 

 base, and subelliptical in outline. First radial pieces larger than the basal, 

 wider than high, nearly quadrangular, and widening moderately from below 

 upwards; facet for the reception of the second radial one third as wide as the 

 summit, and extending down about one fourth the length of the plates, con- 

 cave and sloping outwards, with a deep notch within. Second radial pieces 

 very small, but filling the cavity in each of the first radials, from which they 



[Aug. 



