258 PALAEONTOLOGY OF ILLINOIS. 



Gesus CLADODUS, Agassiz. 

 Cladodus exilis, St. J. and W. 



PI. I, Figs. 1-6. 



Teeth very small and symmetrically formed. Base elliptical in out- 

 line, inferior surface moderately excavated with a relatively broad bev- 

 eled belt along the posterior margin, anterior surface constricted along 

 the basal line of the crown and considerably depressed in the middle or 

 just beneath the principal cone, terminating below in a rather prominent 

 mesial ridge, which is sometimes faintly sinuous in front, and extending 

 one-third to oue-half the lateral diameter of the base, with well-defined 

 lateral angles ; the upper surface of the posterior portion of the base 

 presents a well marked pad or ridge, in lateral extent corresponding 

 nearly with the anteroinferior mesial ridge, gradually sloping to the 

 posterior edge, and separated from the crown by a narrow distinct 

 intervening channel, or, in worn specimens, the surface presents a more 

 or less regular convexity in the region of the pad. The crown consists 

 of a strong, erect or slightly deflected, gradually tapering and slightly 

 sigmoidally curved median cusp, more compressed in front than behind, 

 slightly compressed laterally with sharp cutting edges ; lateral denti- 

 cles normally two on either extremity, slender and tapering to a sharp 

 point with distinct cutting edges, the inner pair about half the size of 

 the exterior pair, and ornamented with numerous exceedingly delicate, 

 closely arranged parallel strise, the same as appear in the median cone, 

 the striae of the inner face being appreciably finer than those in the 

 opposite face, in both faces becoming obsolete just before reaching the 

 lateral edges, and ordinarily extending nearly to the apices in front. Iu 

 rare instances three pairs of lateral denticles have been observed in 

 teeth which otherwise differ in no particular from the normal condition 

 described above. The largest specimens observed attain a lateral diame- 

 ter of base of .00 inch, and from that size down to .15 inch, with a 

 hight of .IS. 



Although not numerously represented, we have had opportunity, 

 through the zealous interest of Messrs. Springer and Wachsmttth, to 

 examine a fine series of this elegant little form, of which there are 

 several nearly perfect individuals. The form differs so markedly from 

 any of its associates in the same beds, as scarcely to require the mention 

 of its distinctive peculiarities. But compared with later succeeding 

 forms, this closely resembles in form, and in many, if not most, of its 

 external details the teeth known under the name C. micropus, N. and 

 T\"., of the Burlington and Keokuk division. The chief distinction con- 



