532 GEOLOGICAL SURVEY OF IOWA. 



Geological formation and, locality. In the Burlington limestone : Bur- 

 lington, Iowa. 



Platycrinus sliimiimlanus ( n. s.). 



PLATE VIII. FIG. 5. 



CALYX somewhat discoid ; base comparatively small, pen- 

 tagonal, without visible suture lines ; margin bevelled, and 

 without this edge the entire plate concave. Radial plates 

 subquadrangular, spreading abruptly outwards below, and 

 turning abruptly upwards at the middle ; edge conspicuously 

 bevelled, leaving the sutures broadly canaliculate : upper 

 margins scarcely concave, the cicatrix of attachment being 

 a broad semicircular scar on the side of the plate, which oc- 

 cupies half its length, and is bounded by a strong thickened 

 margin. Arms dividing upon a large, strong, subbrachial 

 plate which fills the entire articulating space on the face of 

 the radial, each division producing at least three ramifica- 

 tions, making altogether six arms from each ray composed of 

 a double series of thin plates which are wedgeform at their 

 interlocking edges. Tentacula undetermined. 



SURFACE of plates marked by irregular rounded, elongated 

 or confluent nodes, which are often arranged in lines paral- 

 lel to the margins of the plates. The arm-plates are marked 

 by longitudinal rounded ridges. Column of medium size, 

 composed near the body of alternating thicker and thinner 

 joints. 



The calyx of this species resembles in some degree that of P. discoideus ; hut it 

 is proportionally smaller, and the radial plates are higher, while the bevelled surfaces 

 are less broad, and the inner margin on the basal plates marked by a row of small 

 nodes. The nodes of the surface are often elongated, and of irregular form, and al- 

 ways abruptly elevated from the surface. It is readily distinguished from P. wortheni 

 by the character of the surface, as well as form of radial plates which are much 

 thicker, and by the number of arms and narrower arm-plates; while the simple, sub- 

 dued, thread-like lines marking the arm-plates contrast strongly with the sharp 

 spinose elevations on the arms of that species. 



The specimen figured is somewhat distorted at the base of the arms by pressure, 

 while these appendages are all converged together at the summit. 



