230 PALAEONTOLOGY OF ILLINOIS. 



Palaechinus burlingtonensis, M. and W. 



PL 16, Fig. 3 a, 3 6, 3 c. 



Palxchinus burlingtonensis Meek and Worthen, Sept., 1860. Proc. Acad. Nat. Sci., 

 Philad., p. 396. 



The only specimen of this species we have yet seen, is too 

 imperfect to show the exact form of the entire fossil, though it 

 seems to have been nearly spherical in outline. It appears to 

 have only four ranges of interambulacral plates in each area, 

 though it is probable there are at least five, nearer the middle 

 than they can be traced in our specimen. At the extremities 

 of the interambulacral areas the number of plates is reduced 

 to the two lateral ranges of pentagonal pieces, all of which 

 have their truncated margins crenulated for the reception of 

 the outer ends of the small ambulacral pieces. 



The ambulacra are narrow, or about as wide as the first 

 range of interambulacral plates on each side, and slightly con- 

 vex along the middle. They are each composed of a double 

 alternating series of very small pieces, which are two or three 

 times as wide as long. About five to seven of these equal the 

 height of each contiguous interambulacral plate, while the two 

 pores in the outer ends of each piece are arranged so as to form 

 two double, somewhat zigzag, rows in each ambulacrum. 



The surface of all the plates is ornamented by numerous 

 granules, or bases of attachment for the small spines, from 

 thirty to forty of which occupied each of the interambulacral 

 plates, and two or three each of the ambulacral pieces. 



As near as can be determined from our imperfect specimen, 

 it must have been, when entire, not less than 2.25 inches in 

 height, and near two inches in breadth. About twelve of the 

 ambulacral pieces occupy a space of 0.25 inch. 



