VERTEBRATES. 117 



a line connecting the angles of the inner margin, or very slightly 

 oblique in its forward and outward course, the coronal enamel form- 

 ing a shallow fold inbeveled to the channeled basal portion ; postero- 

 lateral border not shown ; inner margin produced obliquely backward 

 and rather sharply rounded at the base of the coronal ridge, thence 

 making a concave curvature to the acute posterior extremity. Cor- 

 onal ridge nearly median, culminating in a rounded crest, posterior 

 slope abrupt to the depression on that side, whence steeply rises 

 the posterior alation, also steeply sloped in front and defined by a 

 faint angulation from the relatively plain belt occupying the anterior 

 portion of the crown, and which is apparently destitute of longi- 

 tudinal furrows or transverse rugae. Coronal surface regularly and 

 closely punctate. Transverse diameter across the inner margin 15.5 

 mm. ; length along antero-lateral border 9.5 mm. 



The above description is based upon a single imperfect specimen 

 derived from the Lower Burlington limestone, and which is the 

 earliest discovered representative of the genus. The somewhat dis- 

 tinct demarkation of the plain anterior area of the crown offers a 

 character recalling typical Cochliodus, its relative great breadth con- 

 stituting the chief distinguishing feature; on the other hand, the 

 coronal contour is remarkably similar to that ascribed to Chitonodus, 

 of which we believe it to be a worn example. Its relations to the 

 species so well represented in the collections from the next succeed- 

 ing or Upper Burlington deposits cannot be fully determined ; but 

 the before-mentioned marked definition of the coronal ridge from the 

 plain belt in front presents a striking contrast in contour with the 

 same form of C. Springeri. Not even worn examples of the latter 

 species, in which the transverse and even the longitudinal folds are 

 obliterated, bear any intimate resemblance in the particulars alluded 

 to as characteristic of the present tooth. 



Geological position and locality : Lower Burlington limestone ; 

 Burlington, Iowa. 



CHITONODUS TKIBULIS, St. J. and W. 



PI. VII, Fig. 18-21. 



Teeth of medium size, of which representatives of the mandibular 

 and maxillary posterior forms alone are known. 



Posterior teeth of the maxillaries presenting the usual sub-quad- 

 rilateral outline, strongly arched and inrolled along the outer margin. 

 Antero-lateral border forming nearly a right angle, with a direct 



