REVISION OF PALEOZOIC STELLEROIDEA. 75 



large radial plates, but inside of the proximal supramarginals there 

 is no basal interradial plate as in Hudsonaster. 



Madreporite unknown. 



Inframarginal and adambulacral plates as in Hudsonaster, except- 

 ing that two of the former and a single axillary plate are in each 

 interbrachial area. Oral armature so far as preserved adambulacral, 

 consisting of five pairs of basal adambulacrals. 



A single, pentagonal, axillary interbrachial plate is present in 

 each area, orad to the two axile inframarginal plates. 



Ambulacral grooves deep and almost closed by the adambulacral 

 columns. Ambulacral plates carinate and apparently one to each 

 adambulacral ossicle. Their relation to one another is not definitely 

 determinable, but apparently they are slightly alternate or nearly 

 opposite. 



Genoholotype. Palxaster shafferi Hall. 



Distribution. The species of Mesopalseaster are restricted to 

 America in the upper portion of the Ordovicic and the lower portion 

 of the Siluric. A species doubtfully referred to this genus occurs 

 hi the Lower Devonic of Germany, and another in the Upper Devonic 

 of Maryland. 



The following are the species of Mesopalseaster: 



M. shafferi (Hall). Middle and upper beds of the Cincinnatic 

 series. Possibly also in the Utica formation of New York. 



M. intermedius, new species. Maysville formation. 



M. finei (Ulrich) . Eden formation. 



M. (?) lanceolatus, new species. Utica formation. 



M. proavitus, new species. Eden formation. 



M. (?) wilberanus (Meek and Worthen). Richmond series. 



M. (?) dubius (Miller and Dyer). Upper Trenton. 



M . (?) antiquus (Troost) . Upper Trenton. 



M. (?) parviusculus (Billings). Siluric (Lower Arisaig). 



M. (?) cataractensis, new species. Siluric (Cataract formation). 



M. granti (Spencer). Siluric (Cataract formation). 



M. bellulus (Billings). Siluric (Rochester shale). 



M. caractaci (Gregory). Caradoc of England. 



M. (?) acuminatus (Simonovitsch) . Lower Devonic of Germany. 



M. (?) clarki (Clarke and Swartz). Upper Devonic (Jennings 

 formation) . 



Remarks. Mesopalseaster apparently developed directly out of 

 Hudsonaster in that the single axillary marginal plates are in the former 

 genus in nearly all the species pushed inward (interbrachial) and 

 their former place occupied by the proximal plates of adjoining 

 inframarginal columns. The same generic feature also distinguishes 

 Mesopalseaster from Palseaster. Further, in Hudsonaster there are 

 no abactinal accessory ray plates, but in Mesopalseaster one or two 

 more or less completely developed columns are inserted on each side 



