250 . PALAEONTOLOGY OF ILLINOIS. 



traced in our specimen; the two outer rows of pentagonal 

 pieces continuous, while four of the intermediate rows of hexa- 

 gonal plates* terminate before reaching either extremity. 



Ambulacral areas half as wide as the interambulacral spaces, 

 and nearly as convex in the middle ; while their broad furrow 

 on each side of this mesial ridge is as deep as those of Melonites 

 rnultipora. Ambulacral pieces somewhat irregular, in conse- 

 quence of the intercalation of a few supplementary pieces, 

 all wider than high, the breadth being from one and a half to 

 seven times the height; those of the inner two rows a little 

 wider than the outer two, and more irregular in form; 

 usually from three to five of those of the outer two ranges occu- 

 pying the vertical space of each contiguous interambulacral 

 .plate; intercalated pieces "apparently always between those of 

 the middle two ranges, but rarely extending into the mesial 

 zigzag sutures. Ambulacral pores circular, two to each piece, 

 and so arranged as to form four vertical ranges, the outer two 

 ranges being more or less zigzag. 



Entire surface occupied by regularly disposed granules, evi- 

 dently the bases of small spines. Of these granules, about 

 twenty-five may be counted on each of the larger interambu- 

 lacral plates, and as many, in proportion to size, on each of the 

 smaller ones, including the ambulacral pieces. 



Our specimen is too imperfect to give exact measurements, but it indicates a 

 height of about 4 inches, and a breadth of probably from 3J to 4 inches below 

 the middle. The greatest breadth of the interambulacral space is about 1£ 

 inches, while the interambulacral plates agree in size and form with those of 

 Melonites rnultipora. 



Specifically this form will be at once distinguished from Melonites rnultipora, 

 even in fragments, where the arrangement of the ambulacral plates can be 

 seen, by their being but four ranges of these pieces, instead of twice that num- 

 ber. (See cut on p. 248.) It also seems to have differed in form somewhat, 

 being apparently proportionally broader below the middle than above. 



* The terminal plate of these intermediate rows, usually differs from the others in 

 being pentagonal, but not from the truncation of one side, like those of the marginal 

 rows, on each side of the ambulacra. 



