440 PALEONTOLOGY OP ILLINOIS. 



least one and three-fourths inches in length, near .20 inch in greatest 

 diameter, and about .14 at the thickest part near the base. 



This pretty little ichthyodorulite bears a striking likeness to Asterop- 

 tycliius f tenuis, of the Chester limestone, but is distinguished by its 

 apparent shorter and stouter proportions and more closely approxima- 

 ted costse, and the broad plane space either side the anterior margin. 

 The intercostal tubercles occur very sparsely ; in the fragment described 

 there are traces of only two or three nodes in the first and second spaces 

 from the anterior border. 



Position and locality : Discovered by Ales. Butters, in the roof 

 shales of coal No. 5, Coal Measures; Carlinville, Illinois. Also in the 

 Lower Coal Measures near Fort Dodge, Iowa. 



Genus GEISACAtfTHUS, St. J. and W. 



Ichthyodorulite more or less curved posteriorly ; anterior angle occu- 

 pied by a simple raised border or keel ; lateral faces ornamented with 

 tubercles arranged in parallel longitudinal rows; anterior keel and 

 tubercles enveloped in a polished enamel-like layer. Posterior face 

 longitudinally keeled. 



The spines under consideration bear striking resemblance to peculiar 

 ichthyodorulites found in the Triassio of Europe, to which Professor 

 Agassiz applied the name Nemacanthus, indicating two species from the 

 "bone-bed" vicinity of Bristol, England. But the American forms 

 may be distinguished by the apparent absence of hooked denticles along 

 the postero lateral angles. Besides, when we recall the faunal associa- 

 tion of the Triassic Nemacanthi, as compared with that of the present 

 form, it seems hardly probable they appertain to one and the same 

 genus, since the ichthyic assemblage in the two respective geological 

 formations bears in no instance generic identity among the class to 

 which these spines belong. Our data is too meagre to warrant even a 

 conjecture as to the probable specific identity of these spines with other 

 imperishable remains of the class described from the same geological 

 position. The typical examples belong to the Chester and St. Louis for- 

 mations, or upper members of the Lower Carboniferous. 



Geisacanthus stellatus, St. J. and W. 



PI. XXI, Fig. 10. 



Pin-spine of small size, very slightly curved, gradually tapering to the 

 apex. Anterior ridge relatively small, sharply rounded along the edge, 

 laterally compressed, and marked by delicate, impressed, oblique stria?, 



