17 



high ; third radials about as large as the second, having upon each of 

 their upper sloping sides three supra-radials,. and upon these the thin, 

 broad brachials rest which complete the edge of the calyx ; the 

 arms bifurcating at about their point of junction with the body. 

 Dome broadly convex, composed of numerous small, tumid plates. 

 Inter-radial series consisting of about fourteen plates ; anal series 

 twenty or more. Surface granulose-? with distinct grooves at the 

 sutures of the body plates. 



From M. Evansi and M. crassus, it may be distinguished by its 

 convex base, prominent basal plate, channeled sutures, and the 

 proportion of the body plates. 



Locality and position, in the upper division of the Burlington 

 Limestone, Burlington, Iowa. — Cabinet of Mr. Charles Wachsmuth. 



Megistocrinus crassus (n. s.) Body large, plates massive, base 

 rather broad, much depressed ; basal plate thick, articulating facet 

 of the column large, slightly concave, from near the margin of 

 which the plate is abruptly bevelled to the sutures between it and 

 the first radials ; first radials very massive, much elevated from the 

 general surface, with nearly perpendicular sides, leaving deep 

 channels at the sutures, somewhat regularly hexagonal, with the 

 longest side adjoining the basal plate, a little wider than high, with 

 a few very coarse, irregular corrugations on their upper surfaces ; 

 second radials hexagonal, about as wide as high, widest above, not 

 so large and thick as the first radials, tumid in the centre ; third 

 radials about half as large as the second, wider than high, much 

 convex. All the plates of the lower part of the body are very thick, 

 and have their sides sloping abruptly to the sutures, while higher up 

 they are more or less regularly convex to the sutures. This species 

 is readily distinguished by its extremely massive plates and deep 

 sutures. 



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

 stone, Burlington, Iowa. — Collection of Mr. Charles Wachsmuth. 



Genus Platycrinus MiUer. 

 Platycrinus pleurovimenus (n. s.) Body broadly cup- 

 shaped, with broad bevelled depressions at the sutures ; dome 

 elevated, and covered with numerous tumid plates ; basal plates 

 anchylosed, forming a broad, pentagonal, concavo-convex disc, 

 with a moderately deep depression in the centre, in which is 

 sometimes observed a still deeper, nearly circular excavation for 

 the attachment of the column ; a broad groove marks the line of 

 the sutures, but does not extend quite to the margin. An Incipient 

 ridge runs from the inner margin of the central depression to each of 

 the angles of the disc ; first radials massive, about one-quarter wider 



PKOCEEDINGS B. 8. N. H.— VOL. IX. 2 APKIL, 1862. 



