130 PAL/EONTOLOGY OF ILLINOIS. 



arched backward, the whole tooth moderately arched vertically and 

 the crown-face vertically convex; the outer face slightly concave 

 vertically, swollen in front of the summit, round which the basal 

 line arches from its slightly concave course near the lateral extremi- 

 ties, which latter are somewhat angularly rounded. The inner and 

 outer coronal margins are sharply inbeveled to the coarsely-pitted 

 basal plate, which behind forms a wide area nearly in the plane of 

 the inner crown-face. The surface punctation is fine, rather widely 

 spaced, much after the same style described in the other forms of 

 the species. The mature inner tooth of the series measures in 

 lateral diameter, 8 mm. ; greatest width from behind outward, 3 

 mm. ; height of inner crown-face, 2.5 mm. The outer or fourth 

 tooth of the same series is 6 mm. in lateral extent, the other dimen- 

 sions proportionately diminished. 



The above described forms embraced under one and the same 

 specific designation, are well represented in the collections accessible 

 to us from the Chester formation. The species is intimately allied 

 to that described from the subjacent St. Louis limestone, Cocldiodus 

 Van Hornii, of which it constitutes a fine example of a represen- 

 tative species. As distinguished from the St. Louis species, the 

 mandibular terminal tooth is relatively narrower in lateral diameter, 

 stronger and more compactly built, the alation proportionately nar- 

 rower and the median-lobe correspondingly wider; the narrow me- 

 dian tooth is apparently more elevated along the crest ; the maxillary 

 posterior teeth, on the other hand, are proportionately more elongate 

 transversely, posterior lobe more prominent, and the median depres- 

 sion more deeply excavated and defined by steeper walls ; in regard 

 to the distinctions observable in the serial teeth, those supposed to 

 pertain to the present species are perceptibly less elongate laterally, 

 with less abrupt inner coronal face, and generally more robust pro- 

 portions, as compared with the normally developed individuals of 

 C. Van Hornii, which, as has been pointed out, vary to a very re- 

 markable degree in their coronal aspects. 



Geological position and localities: Chester limestone ; Chester and 

 Evansville, (Randolph county,) Prairie du Long, (Monroe county,) 

 Illinois, 



