474 PALEONTOLOGY OF ILLINOIS. 



ambulacral areas, 0.76 inch; do. of ambulacra, 0.22 inch; 

 number of ambulacral pieces in 0.10 inch on each side of the 

 mesial suture, 10. 



This species will be readily distinguished from our P. Bvrlingtmensia 

 by the proportionally greater breadth of the ambulacra, and the smaller 

 size, and greater number of its interambulacral pieces, there being sev- 

 en rows of the latter pieces at the widest part of the area, at a point 

 equaling the breadth of these areas in P. Burlingtonensis, where there 

 are only four or five rows of these pieces. Again, its ambulacral areas 

 are equal in breadth to the first and second rows of interambulacral 

 plates on each side, taken together ; while those of P. Burlingtoncnsis 

 are scarcely wider than the single marginal row on each side. 



Locality and position Upper division of the Burlington group, Bur- 

 lington, Iowa. Lower Carboniferous. No. 407 of Mr. WACHSMUTH'S 

 collection. 



Note on the GENUS ONYOHASTEE, M. and W. 



THE borrowed specimens from which we made out the outline cuts 

 and description of this type, published in the third volume of the Illi- 

 nois Geological Report, p. 526, had been, to a great extent, denuded of 

 their outer covering in cutting away the rather hard, firmly adhering 

 matrix, before coining into our hands. Since preparing these cuts and 

 description we have had an opportunity to examine other specimens of 

 this fossil, in part belonging to Mr. WACHSMUTH'S collection, and 

 others among the collections at Springfield, all from the original lo- 

 cality. From these it is evident that, in well preserved examples of 

 this type, the granular outer covering seen on the arms or free rays of 

 some of the specimens first examined, actually covers the dorsal side 

 of the small body also. We likewise observe the presence of a layer 

 of thin, small, imbricating scales under the scattering granules on the 

 dorsal side of the body ; while some specimens certainly show clearly 

 such scales under the granules on the arms. The fact that this granu- 

 lar covering, and this layer of thin imbricating scales, overspread the 

 dorsal side of the body, w r ould certainly show that the appearance of a 

 large central, dorsal opening, which we had marked as an " anal f ' 

 aperture, could not be such. The appearance of pores in some of the 

 pieces around the central opening is also deceptive, and due rather to 

 deep pits than to actual perforations passing through these pieces. 



From the specimens we have since had an opportunity to examine, 

 we are led to think it almost certain that the parts seen around the 



