54 PAL/EONTOLOGY OF ILLINOIS. 



laterally, more strongly so vertically, smooth and polished 

 throughout, but showing every where relatively large, evenly 

 distributed pores ; basal angle a prominent ridge opposite pos- 

 terior one without distinct folds ; root nearly as broad and high 

 as the crown, vertically concave anteriorly, convex posteriorly, 

 not divided. 



In its general form this tooth approaches somewhat nearly to the three pre- 

 ceding species, but in the equal height of the basal ridges of the opposite 

 crown surfaces, and in the porous structure of the anterior face it is allied to 

 the Ilclodi, presenting an interesting link between these two genera. 



Figure 14, represents the anterior aspect of a broken tooth ; .figure .14 «, the 

 crown seen from above; and figure 14 b, a central profile section, natural size. 



Formation and locality: Keokuk limestone, Nauvoo, Illinois. 



Chomatodus affinis, N. and W. 



Plate III, Figs. 15, 15 a. 



Teeth laterally elongated, 5-6 times as broad as high, thick 

 and strong; posterior crown face oblong in outline, vertically 

 and laterally concave, upper and lower borders nearly straight, 

 slightly arched together at the ends; basal folds about 4, nearly 

 equal, parallel and distinct, forming a prominent flattened 

 ridge about one-fourth the height of crown face ; anterior face 

 linear in outline, narrowed to a point at each end, half the 

 height of the posterior face, to which it is inclined at an angle 

 of about 60°, slightly concave vertically; base very concave 

 vertically, slightly arched laterally, forming a broad furrow 

 between the root and anterior basal ridge. This furrow is 

 narrower in the centre than at either end, made so by the 

 slight arching upward of the line of insertion of the root, and 

 downward of the coronal ridge; root nearly as broad as the 

 tooth, forming only a prominent ridge not reaching down to the 

 lower line of the posterior coronal band. 



This species is represented in the collection by much worn teeth only. These 

 have a marked similarity to C. angularis from the Coal Measures at La Salle, 



