1 881.] NATURAL SCIENCES OF PHILADELPHIA. 243 



together ; their ventral furrows covered by two rows of alternate 

 pieces. 



Interradials, one to each space, placed between the free rays ; 

 supported by the first radials, but not touching the second 

 primary or any other radial plate. 



Dome elevated, composed of comparatively few and large 

 plates; apical dome plates prominent; the central and the four 

 large proximal pieces nodose or even spiniferous. Radial regions 

 somewhat elevated, constructed of a double row of larger or 

 smaller pieces, alternatelj' arranged, which, decreasing in size, ex- 

 tend to the extremities of the free rays. Interradial spaces 

 occupied by three — rarely five — plates, smaller than the central 

 dome plates and less nodose, but j^et comparatively large, and 

 resting upon the interradial of the calj^x. On the postero-lateral 

 sides, there are several additional plates, supporting the arms. In 

 rare cases, the interradial dome plates of different zones are later- 

 ally connected, and the radial pieces are thereby pushed to near 

 the edge of the disk. 



Anus in form of a short, heavy tube, composed of smooth 

 plates ; abruptly rounded at its upper end ; with opening nearly 

 central (Platycrinus)^ or in form of a simple lateral opening 

 through the vault {Pleurocrinus) , 



Column large and twisted ; composed of rather large joints, 

 which increase in length as they recede from the body : central 

 perforation minute. The column is comparatively short, and 

 toward the base provided with numerous lateral branches, which 

 like the main stem, terminate in thin, almost hair-like tubes. 

 The joints are transversely elliptic, each one being twisted so that 

 the long diameters of opposite faces make an angle with each 

 other; and, the articulation being in the long diameters, a rapid 

 twist is imparted to the whole stem, permitting motion in all 

 directions. The stem seems to be regularly articulated, which is 

 not the case in crinoids with a round or pentagonal stem ; the 

 articulating lines run lengthwise of the faces of the joints and 

 consist of a long ridge along the middle, with deep depressions 

 on either side, which latter were evidently filled by ligament. 

 The column forms one of the most characteristic features of the 

 genus. 



Geological Position, etc. — Plat y annus, as here defined, is almost 

 exclusively a Subcarboniferous genus, only two small species (one 



