VERTEBRATES. 375 



following out its affinities with the preceding forms, it seems to present 

 equally intimate relationship with the latter. The features in common 

 ■with Tanaodus consist in the wedge-shaped outline of the root, the con- 

 vexity of the basal area, and the peculiar coronal contour, characters 

 which present a strong contrast to the typical forms of Antliodus. 



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

 Iowa, Alton, Illinois, St. Louis, Missouri. 



Tanaodus grossiplicatus, St. J. and W. 



PI. XI, Fig. 26. 



Teeth small or medium in size, viewed from above elongate-trapezoi- 

 dal iu outline. Crown culminating along the straight convex face in a 

 low, even-crested ridge slightly produced outward, the basal angle 

 defined by a narrow coronal fold, nearly straight throughout the greater 

 extent, but gently curved upward and downward at the extremities ; 

 concave face descending in a wide, gradual and gently depressed incli- 

 nation to the basal margin, which is defined by a rather prominent coro- 

 nal belt composed of two or three strong imbricating folds, in the median 

 half straight and parallel with the opposite face, more or less suddenly 

 and obliquely truncated in the lateral portions, the coronal belt contin- 

 uous with that in the convex face ; the coronal surfaces are invested 

 with a delicately wrought enamel layer, and in the worn triturating sur- 

 face along the crest a striato-punctate structure appears. The basal 

 area presents in outline very nearly the figure of the concave crown- 

 face, nearly plane or laterally traversed by a faint ridge ; the root is 

 placed close beneath the concave margin, produced downward and out- 

 ward in nearly the same plane as the convex crown-face, which it nearly 

 equals in depth, moderately strong, in transverse diameter considerably 

 less than the crown, inferior surface more or less well defined and bev- 

 eled in the horizontal plane of the crown. 



The single representative of the form above described, unfortunately, 

 preserves only about half the entire lateral diameter of the tooth, which 

 is otherwise in an unusually perfect state of preservation. Iu general 

 form it bears closest resemblance to the Coal-measure form T. angulatus 

 (S. and W. sp.,) from which it differs not only iu its smaller size, but in 

 the less numerous imbrications in the coronal belt of the concave face. 

 Its resemblance to some of the Keokuk and Upper Burlington congene- 

 ric forms is also quite striking, yet it is distinguishable by its straight 

 concave border, even crest, and stronger proportions. 



Position and locality : Probably from the lower fish-bed of the Chester 

 formation ; Chester, Illinois. 



