158 PALEONTOLOGY OF ILLINOIS. 



The above diagnoses are based upon examples of the mandibular, 

 posterior and median forms, and the posterior form of the tipper 

 jaw, derived from lower Coal Measure strata of Illinois. The species 

 is intimately allied to that occurring in the Chester formation, Del- 

 todus cingulatus, N. and W. The mandibular terminal form, how- 

 ever, is distinguishable by its stouter proportions, wider coronal 

 ridge, and relatively shorter longitudinal diameter. The opposed 

 teeth of the upper jaw, again, differ from the corresponding Chester 

 form in the more prominently transversely arched alation, in which 

 particular it bears closer resemblance to D. undulatus, N. and W., 

 of the Keokuk limestone, from which, however, it widely differs in 

 the more angularly rounded contour and concave posterior slope of 

 the principal coronal prominence. The Coal Measure forms of the 

 Upper Mississippi region, therefore, constitute a well defined species, 

 as distinguished from the several specific groups occurring in the 

 various formations of the Lower Carboniferous groups in the same 

 region. Its intimate resemblance to the species described from the 

 (probably) Coal Measure limestone of the Far West, D. Powellii, 

 has already been remarked upon. It is possible that a larger suite 

 of specimens of the latter species might prove its specific identity 

 with the present teeth; but this can only be demonstrated by the 

 comparison of all the forms. 



Geological position and localities: The first example of the present 

 species that was brought to our notice, several years ago, -was dis- 

 covered by Mr. L. A. Fuller in the shales overlying Coal No. 7, at 

 Danville. Subsequently, Mr. Alexander Butters obtained from the 

 roof shales of Coal No. 5, at Carlinville, representatives of all three 

 - of the forms thus far identified as pertaining to this species. 



GENUS DELTODOPSIS,* St. J. and W. 



Teeth possessing coronal contour and general aspect near Ddtodus. 



Mandibular posterior teeth distinguished by their oblique sub-tri- 

 angular outline and the remarkable obliquity of the anterior portion 

 of the inner margin, proportionately short extent of the anterior 

 articular border, the sometimes longitudinally depressed condition 

 of the anterior slope of the coronal prominence, and the tendency 

 to produce a more or less well defined median ridge along its sum- 



*In"allusion to a resemblance to Deltodus. 



