FOSSILS OF THE ST. LOUIS GROUP. 533 



The peculiarity of having the corners of the body plates impressed so 

 as to form large ridges, or prominences on the plates, is very common 

 in this group of erinoids. but it differs in degree, and other details, in 

 the various species in which we find other corresponding differences in 

 other parts. It is more strongly defined in the species here described, 

 and the ridges are more angular than in any other species known to us. 

 This >peries is al>o the first of the group in which we have clearly seen 

 ndary armlets springing from the first, as the latter are given off 

 from each main arm ; but since seeing this character in this species, we 

 think we have seen indications of it in some otli 



J.'-alUy and position Otter Creek, Jersey county, Illinois; from the 

 St. Louis group, of the Lower Carboniferous. 



GEXUS POTERIOCKIXITES. 



ABDtxEXSis, AVortheu Ms. 



PI. 20. Fie 10. 



BODY above the medium .size, urn-shaped, base large, 

 squarely truncated, and entirely covered by the first col- 

 umnar joints. Basal plates pentagonal, wider than high, 

 and but slightly spreading toward their summits. Sub-ra- 

 dials nearly equal in size, four of them hexagonal and two 

 heptagonal. Anal plates two, the first about half the size 

 of the sub-radials. the other much smaller, and both pen- 

 tagonal. The first radials wider than high, except the an- 

 terior one, which is narrower, the width and hight being in 

 that about equal. Arm plates thin and flat, so for as they are 

 shown in the specimen, there being from two to four above 

 the first radials on each ray. Column consisting at the top of 

 very thin, circular plates firmly anchylosed together, and 

 very gradually decreasing in size below its junction with 

 the ha- 



Locality and position Uppermost beds of the St. Louis Limestone; 

 lare. Hardiu county, Illinois. 



