VERTEBRATES. l\ 1 



convex face, broadly and usually regularly arched between the angles 

 of the straight lateral margins, inferior border gently arched down- 

 ward and abruptly defined or inbeveled, and marked by a relatively 

 ■wide coronal band composed of three or more imbrications, which 

 abruptly terminate at the inferior angles of the lateral margins. The 

 coronal surfaces present a fine granulopunctate structure. The infe- 

 rior or basal area is nearly equal in extent to the concave crown- 

 face, gently convex laterally, smooth and well-defined from the convex 

 crown-face; the root forms a trowel - shaped process projecting from 

 the lower border of the basal area, the anteroposterior diameter of 

 which it nearly equals in its outward prolongation, slightly contracted 

 at the shoulder and rather deeply excavated, rapidly tapering to an 

 obtusely rounded point, aud plane below. Length and lateral diameter 

 of crown nearly equal, the largest tooth measuring .35 inch, smaller, 

 teeth .16 inch in length and breadth. 



This elegant little species is known to us by several individuals, the 

 first having been discovered by Mr. Van Horne, at Alton, and others 

 subsequently obtained at St. Louis, all from the same horizon in the St. 

 Louis limestone. In general form it bears a somewhat close resem- 

 blance to P. transversus from the Coal Measures. Its quadrangular 

 outline as seeu from the concave side, and prominent coronal imbri- 

 cations, however, will readily distinguish the present form from that 

 .mentioned above, which it is even more strongly marked in contrast 

 with Peltodus unguiformis of the Upper Goal Measures. 



Position and locality: Upper beds of the St. Louis limestone; Alton, 

 Illinois,"and St. Louis, Missouri. 



Peltodus ? plicomphaltjs, St. J. and W. 



PI. XDT, Fig. 8. 



Teeth small, subovate in outline. Grown moderately thick, crest sub- 

 acute, rather strongly arched, acuminate, or truncated from wear; con- 

 cave crown-face broadly ovate, antero-posterior diameter equal to, if 

 not exceeding the lateral diameter, lateral margins gently rounded, 

 median line occupied by a somewhat prominent vertical fold, which is 

 flanked on either side by nearly parallel though obscurely defined plicse, 

 intermediate lateral portions slightly depressed vertically, basal margin 

 unknown — probably broadly arched downward, and bordered by a nar- 

 row, irregularly imbricated coronal belt; convex face sublunate in out- 

 line, relatively low, gently arched laterally, slightly depressed in the 

 middle towards the base, where it is defined by a well-marked coronal 

 fold, which is abruptly arched downward in the middle and strongly 



