REVISION OF PALEOZOIC STELLEROIDEA. 145 



mally they are separated from each other by numerous accessory 

 interbrachial plates, of which there appear to be not less than 30 in 

 each area. The apexes of these interbrachial areas form the oral 

 armature and each consists of two adambulacral plates, back of 

 which is a rather large interbrachial plate. 



Ambulacra! grooves narrow, tapering, and deeply V-shaped. 

 Ambulacral plates slightly alternating, about as long as wide, and 

 proximally superposed, with about 26 in each column. Each plate 

 has an L-shaped ridge with the point proximally directed. The 

 podial openings are situated in the outer proximal corner of the 

 plates and between the inner angles of two adambulacrals. 



Formation and locality. Richmond and Maysville formations of 

 the Upper Ordovicic. The holotype of P. speciosa was found by 

 Mr. W. C. Barnhart, on Twin Creek, near Winchester, in Preble 

 County, Ohio, and is now in the Dyer collection of the Museum of 

 Comparative Zoology, Harvard University (No. 14). Four other 

 less well-preserved specimens are in the Harris collection of the 

 United States National Museum and were found in the vicinity of 

 Waynesville, Ohio. The holotype of P. approximata and another 

 similar-sized specimen but not so well preserved are also from the 

 vicinity of Waynesville, Ohio. The first is in the Dyer collection 

 of the Museum of Comparative Zoology (No. 15), and the other in 

 the United States National Museum. There is a slab with two 

 young individuals much etched with acid but otherwise excellent, 

 showing the actinal side, in the Gurley collection of the University 

 of Chicago (No. 10979). Finally there are two fine specimens, not 

 fully grown, showing well the actinal and abactinal sides, in the 

 Haines collection of the same university (Nos. 10839 and 10840), 

 found near Hamilton, Ohio; these are apparently from the very top 

 of the Maysville formation. 



Remarks. P. speciosa is described by Miller and Dyer from the 

 abactinal side and P. approximata from the actinal side. These 

 writers point out that the rays are more slender and the disk more 

 contracted in the latter, but when one considers that it is less than 

 half the size of the former and that during growth these forms con- 

 tinually add accessory plates, this difference hi form is seen to be 

 one of growth. The ' 'smaller dorsal plates" noted by these writers in 

 Mrs. Haines's specimen of P. approximata are a character which the 

 present writer does not regard as of value unless they be constant 

 at this lower geologic horizon (Maysville at Hamilton), in which event 

 this specimen can later on be distinguished by another specific name. 



Cat. No. 60609, U.S.N.M. 



