106 PALAEONTOLOGY OF ILLINOIS. 



The above diagnoses are based upon unique examples in each 

 instance, the former preserving scarcely more than the outline and 

 the excessively worn superficial contour, the latter showing only the 

 central portion of the tooth, the mutilated borders not displaying 

 the outline of the tooth. They furnish a singular instance of the 

 paucity of certain fish-remains in the collections from a formation 

 which has yielded rich and varied materials pertaining to allied 

 genera. The teeth certainly, so far as it is possible to carry the 

 comparison, offer striking resemblance to the corresponding forms 

 occurring in the superjacent Warsaw limestone, Stenopterodus elon- 

 gatus, in outline, apparently coronal conformation, and the character 

 and disposition of the punctae. We should, therefore, be prepared 

 to recognize their specific identity with the latter forms were the 

 characters noticed in connection with the sole examples as yet 

 known to us from the Keokuk horizon, shown to be persistent and 

 normal in even a small suite of perfect specimens. 



Geological position and locality : Keokuk limestone, fish-bed ; War- 

 saw and Hamilton, Illinois. 



STENOPTERODUS ELONGATUS, St. J. and W. 



PI. IV, Fig. 1-3. 



Maxillary posterior teeth small, elliptical in outline, considerably 

 arched and strongly inrolled along the outer margin. Course of 

 antero-lateral border nearly at right angles to the transverse axis of 

 the tooth, opposite border gradually converging from the obtusely- 

 pointed extremity toward point of inrollment, neither border 

 sufficiently well preserved to show details of enamel fold and basal 

 rim. Coronal prominence relatively very broad, or occupying two- 

 thirds the lateral diameter of the tooth, abruptly beveled along the 

 postero- lateral border, broadly and regularly arched transversely, 

 culminating anteriorly where it is defined by a slight declivity hav- 

 ing a very oblique course toward the outer extremity ; anterior area 

 relatively narrow, quite smooth, plane, and nicely rounded in a 

 narrow fold along the border, which is defined by a narrow sulcus 

 from the shallow basal rim. Coronal surface minutely and finely 

 punctate, the punctae exhibiting a somewhat marked tendency to 

 linear longitudinal disposition, and in mature examples the enamel 

 shows distinct parallel lines of growth following the broadly rounded 

 curvature of the inner margin. Greatest diameter of a small-size i 

 tooth, 15 mm. ; greatest diagonal breadth near middle, G mm. ; 

 length along antero-lateral border, 4 mm. 



