100 PALEONTOLOGY OF ILLINOIS. 



teeth, which probably belonged to the same species. The latter, 

 however, are in so worn and mutilated a state of preservation as 

 not to exhibit details beyond mere outline, by which they may be 

 satisfactorily compared with other specimens of the same form. 



The loan of the type was kindly procured us by Dr. Leicly, a 

 careful comparison of which with the material before us leaves not 

 the least doubt as to their specific identity. The species, as repre- 

 sented by the posterior teeth of the mandible, is intimately allied to 

 D. expansus, of the St. Louis limestone, being chiefly distinguished 

 by its more -robust figure, stronger inrollment, and narrower pos- . 

 terior alation. The majority of the examples of this form exhibit 

 traces of excessive coronal abrasion, which in some instances has 

 leveled the median prominence, destroying its definition from the 

 plane anterior belt, in which latter condition the individuals bear 

 deceptive resemblance to the homologous teeth of Orthopleurodus. 



Geological position and locality: Chester limestone ; Chester, 111. 



GENUS STEISTOPTEKODUS, 1 St. J. and W. 



Teeth, probably occupying a posterior position on the upper jaw, 

 distinguished by their long-elliptic outline, strongly arched and spiral 

 inrollment of the outer extremity. Crown traversed by a posterior 

 prominence or lobe in the direction of inrollment, more or less well- 

 defined from the anterior area, which presents a plane surface, 

 without prominent revolving or longitudinal folds; posteriorly, the 

 crown is well defined from the base by the inbeveled enamel fold, 

 beyond which the basal rim usually more or less projects, often 

 terminating in a strong posterior process, and forming, in well- 

 preserved specimens, a deep border toward the inner margin, becoming 

 less conspicuous outwardly ; anteriorly truncate, usually more or less 

 obliquely so from the inner angle outward and backward to point of 

 inrollment (though sometimes outward and forward from an obtuse 

 inner angle), the shallow enamel fold inbeveled and defined by a 

 narrow sulcus from the base. Surface of crown in perfect state 

 covered by a dense enamel-like, glossy coating, through which the 

 tubular structure presents a delicate papillose appearance; worn 

 surfaces minutely and densely pitted by the exposed extremities of 

 the medullary tubes ; vaso-dentine of base coarse, inferior surface 

 of the dense inferior layer irregularly striated in the direction of 

 inrollment. 



NOTE 1 Ste.noK, narrow: pteron, wing; odous, tooth. In allusion to the narrow 

 alation of the mandibular posterior tooth 



