than high, the sides of which extend with an obtuse angle above the 

 articulating arm-facet. This is quite prominent, subcircular, occupying 

 about one-third of the width of the plate, with a narrow, deep notch 

 in the centre of the upper sides ; surface of the broad bevelled edges 

 of the plates marked by parallel ridges, and the surface between these 

 and the articulating facet faintly and coarsely corrugated. Arms 

 strong, broadly rounded on the back, bifurcating upon the second ra- 

 dial, yt't not separating from each other immediately, but continued 

 as a compound arm, composed of three pairs of strong plates which 

 gradually increase in width, and abut obliquely against each other by 

 their inner edges, except the third pair, which abut by only about one- 

 half their height, and from which the arms diverge, being composed of 

 fourteen or fifteen other plates, broad and strong, extending clear 

 across the back of the arm, with straight or slightly oblique sutures, 

 the articulating faces of which are crenulated. Beyond this, they 

 bifurcate again. A series of sma^l plates arch over the upper side of 

 the arms, from the sides of which numerous ai'mlets diverge, which 

 bear the tentacles, and are composed of a double series of plates like 

 the usual simple arms of Platycrinus. 



Column subcircular at the upper ends, but rapidly assuming a twist- 

 ed, oval form, and composed of thin joints, gradually increasing in 

 thickness from the body. This species is probably the same to which 

 the large plates belonged which Messrs. Owen and Shumard referred 

 to their P. discoideus^ and may also be the same as that figured by Prof. 

 Hall in the Iowa Reports, and referred also to P. discoideus ; yet the 

 latter presents some important differences from our specimens, and, it 

 is not improbable, will prove to be a different species, as separate plates 

 are frequently found, showing this constant variety. I should not 

 question the conclusions of these eminent men, were it not that lately- 

 discovered specimens of P. discoideus^ as figured and described by 

 IVIessrs. Owen and Shumard, show the arms to have been simple, like 

 those of P. Shumardianus, P. Wortheni, &c., as seen figured in the 

 Iowa Reports, while our species has arms so differently constructed that 

 it may yet be found expedient to consider this difference of generic 

 importance. 



Locality and position, in the upper division of the Burlington Lime- 

 stone, Burlington, Iowa. — My own cabinet. 



Platycrinus qxjinquenodus (n. s.) Body cup-shaped, spreading 

 above, base truncated, having five distinct nodes situated at the outer 

 margin of the truncation, and directly beneath each perpendicular suture 

 of the first radials ; basal plates bent upward at the edges, their slightly 

 concave sides joining the convex ends of the first radials, sliowing a 

 shallow depression at the sutures ; first radials hexagonal in outline, 

 slightly convex, a little depressed at the su'ures ; articulating arm-facet 



