VERTEBRATES. 417 



in subacute, eccentric apices — in some individuals rudimentary lobes 

 appear in either lateral margin below tbe principal cusps, but these 

 are discernible only in exceptionally well preserved specimens, if 

 indeed they are always developed. Unfortunately none of the speci- 

 mens before us preserve the basal portion of the teeth, hence we are 

 lead to conjecture the broadly downward arched basal border of the 

 concave crown-face, and its probable imbricated belt, as also the 

 probable relative diminntiveness of the produced root portion, which 

 latter probably more or less intimately resembles the root in Fissodus 

 and Peltodus, with which these teeth otherwise are intimately related. 

 Lateral diameter of tooth .45 inch, antero-posterior diameter above .CO, 

 thickest portion about .09, greatest hight of convex crown-face .21 inch. 



The distinctly lobed character of the crest of the present form defin- 

 itely distinguishes it from Peltodus unguiformis, K. and W., which 

 occurs in the same horizon, and with which it has one striking feature 

 in common, viz : the general uniformity in the convexity of the convex 

 crown-face and basal region, and the delicate demarkation defining- the 

 basal from the coronal region. But the singular bifid character of the 

 crest strongly contrasts with the regular outline of the crest of Peltodus, 

 and almost equally so with the symmetrical cuspidations of Fissodus, 

 differences which would seem to remove them from the same generic 

 category, unless, indeed, the opposite jaws bore teeth so very dissimilar. 

 However, we have no evidence of a nature that more than vaguely sug- 

 gests such specific relationship, and the non-occurrence of both forms 

 in the Chester and St. Louis formations, both of which horizons have 

 yielded peculiar examples of one or other type, seems further to militate 

 against such identity. 



The present form is, so far at least as our information extends, 

 restricted to the upper division 'of the Coal Measures, examples of 

 which have been found at distant localities in Iowa and Illinois. Some 

 of the Iowa specimens are not quite as strong, and the rudimentary or 

 lateral lobes are more distinctly developed than in the ordinary exam- 

 ples, as shown in the illustrations introduced in PI. xiv, fig. 4. 



Position and locality : Upper Coal Measures, above the horizon of 

 coal Xo. 8, near Springfield, Illinois, and in a similar horizon near Cla- 

 rinda, Iowa. 



Gexus PSEPHODUS, Agassiz. 

 Psephodtjs % BETicuLATTjs, St. J. and W. 



PI. Vr, Fig. 19-24. ' 



Teeth attain large size, ranging from .20 to 1.15 inches in lateral 

 diameter, more or less irregular in outline, massive, depressed or strongly 



