126 PALEONTOLOGY OF ILLINOIS. 



lections afford examples of teeth corresponding to the anterior form 

 described by Professor Owen. 



In relative frequency of occurrence it would, of course, be expected 

 that the larger and more massive teeth appear most abundant in 

 the collections. The posterior teeth of the mandibles and maxil- 

 laries are represented about equally in numbers, while the narrow 

 median teeth of the lower jaw, and the Helodus-like forms presum- 

 ably pertaining to the anterior portion of the upper jaw, are less 

 frequently met with. This is also true of other species of the same 

 and allied genera occurring in the Lower Carboniferous formations, 

 and the absence of certain forms ought not to militate against the 

 conviction of their existence. 



Geological position and localities: St. Louis limestone; Alton, and 

 Monroe county, Illinois; St. Louis, Missouri; Pella, Iowa. 



COCHLIODUS OBLIQTJUS, St. J. and W. 



PL VII, Fig. 17. 



Tooth resembling in coronal contour the form assigned to the 

 posterior position on the upper jaw, very small, obliquely trapezoidal 

 in outline, moderately arched in the direction of inrollment. Antero- 

 lateral border nearly straight, obliquely produced forward from the 

 inner angle, making an angle of about 40 with a right line con- 

 necting the anterior and posterior angles of the inner margin, low 

 basal portion forming a shallow, channeled rim, defined above by 

 the narrow, rounded, inbeveled enamel fold ; postero-lateral or oblique 

 border converging from the rounded posterior extremity to point of 

 inrollment, at an angle of 20, more or less, with the opposite bor- 

 der, basal portion relatively deep, moderately inbeveled and slightly 

 channeled, the coronal enamel forming a delicate fold denning the 

 upper edge; inner margin broadly arched round the base of the 

 postero-coronal lobe, thence to the obtuse anterior angle nearly 

 straight and closely conforming to a line connecting the inner angles 

 of the tooth. Coronal region presenting a strong posterior promi- 

 nence, the broad outer slope rounding into the border, from which 

 it is denned by a slight depressed belt, anterior slope more abrupt 

 and making a slight angulation where it joins the anterior depressed 

 portion of the crown ; the latter presents an area of about the same 

 breadth as that occupied by the posterior lobe, depressed, and 

 marked by two faint, revolving folds, with a third stronger ridge 



