410 PALEONTOLOGY OF ILLINOIS. 



■whose concave face occupies the greater extent of the tooth, while the 

 opposite face, instead of the strong definition in the typical genera of 

 the family, forms a more or less regular convexity with the basal area 

 (not the "crown surface"), from which, however, it is defined by a 

 distinct though very narrow fold or thickening of the coroual border, 

 as shown in the figures above cited, and lunate in general outline; 

 the concave crown-face is gently depressed, broadly oval or sub-quad- 

 rate in outline, the direction of the greater diameter varying in differ- 

 ent species, but usually vertically elongated, crest broadly arched, 

 subacute, lateral margins sometimes more or less parallel or slightly 

 converging towards the base, as in the above typical species, basal 

 margin gently arched downward and generally forming an angle at 

 the lateral margins, abruptly truncated or beveled, and bordered by 

 an imbricated belt, as in Petalodus, etc. The inferior or basal area is 

 similar in outline, though less in superficial extent than the concave 

 crown-face, moderately convex; the root arises in the lower border, 

 projecting slightly downward and suddenly outward, gradually tapering 

 to a rounded point, vertically flattened, narrow, excavated in the inner 

 side of the shoulder, and trowel-shaped. Both crown faces are invested 

 with an enamel-like layer, sometimes more or less rugose; the basal 

 area is quite smooth, and the root proper more or less roughened. 



All the above mentioned characters are present in the original speci- 

 mens of P. unguiformis, even the root is shown in the above oiced 

 figures, though its extremity is imperfect, having been broken away. 

 By reference to the figures of the following described forms, it will at 

 once be apparent that the genus is a true Petalodont, the teeth holding 

 the same position upon the jaws as do those of Petalorhynchus, Antliodus, 

 etc., etc., from which they chiefly differ in the less erect crest and the 

 peculiar shape of the root. 



According to our present knowledge, the genus has no representa- 

 tives below the middle or St. Louis division of the Lower Carboniferous 

 series, extending into the Upper Coal Measures. 



Peltodus qtjadrattjs, St. J. and W. 



PI. XIII, Fig. 8, 7. 



Teeth of small size, quadrangular in outline. Convex crown-face 

 low, broadly arched laterally, basal border forming an obtuse angle 

 with the basal area, somewhat strongly arched downward in the mid- 

 dle, broadly and regularly curved to the lateral borders, and defined 

 by two or more delicate imbricating folds; general contour of the 

 concave face moderately depressed, flattened or plane below, crest 

 rather sharp and projecting outward beyond the basal margin of the 



