DISCOVERY OF THE DISK OF ONYCHOCRINUS 477 



These observations suggested also the idea that, if this should 

 prove to be the fact in the Silurian and not in the Carboniferous 

 species, then we should have a further and most interesting example 

 of evolution in the elimination of the radianal from Silurian to Car- 

 boniferous times, within what has been considered as a single genus. 

 With a view to ascertaining the facts deiinitely, I undertook a care- 

 ful examination of my specimens, with additional preparation, some- 

 times grinding off part of the base to expose the infrabasals; and I 

 have also since then had in hand for study good specimens from the 

 Niagara of Canada and New York, most obligingly loaned me by 

 Mr. Byron E. Walker, of Toronto, and Dr. E. N, S. Ringueberg, 

 of Lockport, N. Y. This examination gave the following results : 



Silurian Species 



I. pyrijormis ( ?), from the Wenlock Limestone, Dudley, England: 

 In a large and quite perfect specimen I removed the stem, and by 

 grinding succeeded in exposing the infrabasals; along with which 

 appeared a radianal under the right posterior radial. This is invisi- 

 ble in a side view, but appears on the flat part of the base, only half 

 as large as the radials in the other four rays (Plate IV, Fig. 3). An- 

 other specimen of true /. pyrijormis in my collection, and three others 

 since examined in the British Museum, show the radianal. 



/. laevis, from the Niagara: Eight specimens from New York 

 and Canada show infrabasals and radianal (Plate VI, Fig. i). 



/. suhangularis,-irova the Niagara: Five specimens from Indiana 

 and Illinois show infrabasals and radianal (Plate VI, Fig. 2); and 

 numerous others among the casts of /. corhis found at Milwaukee 

 and Chicago which do not preserve the infrabasals, distinctly show 

 the impressions of the radianal. 



Carboniferous Species 



I. burlingtonensis, Lower Burlington Limestone: Six specimens 

 preserving the rays all around show uniformly a radial and two 

 primibrachs, without any sign of a radianal (Plate VI, Fig. 15). 

 Two of them have the infrabasals well preserved, one both exter- 

 nally and internally. 



/. tiaraejormis, Keokuk Limestone, Tennessee: Besides the type, 



