248 



PALAEONTOLOGY OF ILLINOIS. 



Fig. 21. 



series of interambulacral plates, all in very nearly their natu- 

 ral position, excepting that the specimen is somewhat flattened 

 by pressure. We at first referred it to the genus Melonites, but 

 subsequently proposed to separate it, at least subgenerically, 

 from the type of that genus, and later comparisons now lead 

 us to regard it as generally distinct. So far as its characters 

 can be made out, it seems to agree almost exactly with Melon- 

 ites in size, form, the number and thickness, as well as the 

 arrangement of its interambulacral plates, and in the breadth 

 and deeply double furrowed character of its ambulacra. It 

 differs, however, in the important peculiarity of having only 

 four rows of ambulacral plates, and four double rows of pores, 



to each ambulacrum, in- 

 stead of about ten of 

 each. This difference 

 will be more clearly un- 

 derstood by comparing 

 the annexed cuts — No. 



Melonites multipora. (Two diameters ) 



Diagram showing the number and arrangement of the 27 representing the HU1H- 

 rows of ambulacral pieces and pores, and the connection of tr ' ' O ' 



the former with the interambulacral pieces on each side, Vvov of vrraro nnrl orrnnn'a 

 near the middle of the ambulacral area. ut;l ui J-UVVfc <mu. dd I cillgti- 



ment of the plates and pores 

 in the ambulacra of Melonites 

 multipora, and the cut No. 28 

 the same in Oligoporus Danes, 

 both from near the middle of 

 the ambulacral series. Be- 



Oligoporus Dance. (Two diameters.) & I J 



Diagram showing the ambulacral pieces and pores, the form Ullder Consideration 



and the connection of the former with the first range 



'^'a^SbSS^^S^'^^ 1 ^^ 11 ^^'^ have been accompanied 

 by other equally important differences in the apical disc, and 

 the structure of the softer parts of the animal, we can but 

 regard it, with our present means of comparison, as the type 

 of a distinct genus. 



It will thus be seen that the type of Oligoporus seems to be 

 almost exactly intermediate, in some of its characters, between 



