82 PROCEEDINGS OF THE ACADEMY OF 



PALiECHiNus GRACILIS, M. and W. 



Body small, and apparently oval or subglobose. Interambnlacral areas a 

 little convex. Interambnlacral plates in seven rows at the middle, but appar- 

 ently only the marginal rows of pentagonal pieces are continued to the upper 

 and lower extremities of the areas, the intermediate hexagonal pieces running 

 out at various distances between the middle and the ends of the areas; thick- 

 ness of each about half the breadth of the largest ; all ornamented with closely 

 set granules, of which 25 to 30 may be counted on each of the larger pieces. 

 Ambulacral areas slightly convex, and equaling in breadth the first and second 

 rows of interambulacral plates on each side ; composed of very short pieces, 

 which are a little thinner than the interambulacral plates, and about three 

 times as wide as long, the widest part of every alternate one being at the outer 

 end, which is received into a little sinus in the adjacent marginal interam- 

 bulacral plate (there being generally three of these little sinuses to each of 

 these plates), while the intermediate pieces usually wedge out rather abruptly 

 before quite reaching the lateral margins; the two pores at the outer end of 

 each piece arranged so as to form two zigzag or undulating rows along the 

 lateral margins of each ambulacral area ; surface of the whole series occupied 

 by granules of the same size as those of the interambulacral plates. 



Entire dimensions unknown ; greatest breadth of interambulacral 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, ten. 



This species will be readily distinguised from our P. Burlingtonensis by the 

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

 greater number of its interambulacral pieces, there being seven rows of the 

 latter pieces at the widest part of the area, at a point equaling the breadth of 

 these areas in P. Burlingtoi^ensis, 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. Burlijigtonemis are scarcely wider than the single marginal row on 

 each side. 



Localily and position. — Upper division of the Burlington group, Burlington 

 Iowa. Lower Carboniferous. No. 407 of Mr. Wachsmuth's collection. 



Note on the Genus ONYCHASTER, M. and W. 



The borrowed specimens from which we made out the outline cuts and de- 

 scription of this type, published in the third volume of the Illinois 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 coming into our 

 hands. Since preparing these cuts and descriptions we have had an oppor- 

 tunity 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 locality. 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 granular covering, and this 

 layer of thin imbricating scales, overspread the dorsal side of the body, would 

 certainly show that the appearance of a large central, dorsal opening, which we 

 had marked as an " anal ?" 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 central opening 



[April, 



