480 PALAEONTOLOGY OF ILLINOIS. 



unequal size and form, and irregularly arranged, most of 

 those starting from the mesial, zigzag suture extending out 

 so as to connect with the crenated, lapping margins of the 

 outer ranges of interambulacral plates, while many of those 

 starting from the latter inward, wedge out more or less 

 abruptly between the others, at various distances, before 

 reaching the mesial suture, so as to present the appearance 

 of a strong tendency to run into two rows of pieces on each 

 side of the mesial suture, each pierced by two pores, which, 

 owing to the irregular arrangement of the plates, present 

 the appearance of forming two double rows along near each 

 lateral margin of each ambulacrum, or four rows to each 

 of these areas. They might, however, with probably almost 

 as much propriety, be counted as one double, strongly zig- 

 zag row on each side. 



Near what appears to be the position of the oral opening 

 there is adhering to the specimen one-half of a stout jaw, 

 0.60 inch in length. On its outer side it is cuneiform, a 

 little arched, and provided with a broad, longitudinal, ex- 

 centric furrow; its lateral margins are smooth, and near 

 0.30 inch in breadth at the base, and converge to a sharp 

 edge within. We have now tolerable good evidence that 

 all the different genera of the Perischcechinidce are provided 

 with strong jaws. 



The specimen is too imperfect to give a good idea of its general form 

 or size. As it shows one of the interambulacral areas, however, to be 

 near two inches broad, it is probable the entire fossil was not less than 

 four and a half inches in its transverse diameter. The largest interam- 

 bulacral plates measure about 0.35 inch in length and breadth, while 

 those of the outer rows next the ambulacra are proportionally narrower. 

 The primary spines seem to be about one inch or more in length, though 

 we have seen none entire. The longest fragments we have seen are 

 about 0.50 inch long, 0.10 inch thickness at the articulating end, and 

 O.OG inch in diameter a little above, but without any taper toward the 

 broken end. The ambulacra! areas are only about 0.26 inch in breadth 

 at the widest place, near the middle, and about four to six of their 

 pieces fit into the creuulations of each of the marginal interambulacral 

 plates. 



