VERTEBRATES. 193 



outer inrolled extremity 14 mm. ; greatest breadth between angles of 

 inner margin 9.5 mm. ; breadth of coronal prominence at inner 

 margin 5 mm. ; length along antero-lateral border 11 mm. Man- 

 dibular median tooth, length along posterior articular border 8 mm., 

 breadth along inner margin 6.5 mm. 



The long posterior teeth of the mandible, which in well-preserved 

 examples show the coronal ridge denned on either side by a slight 

 impressed angulation, owing to the extreme thinness of the outer 

 portion are invariably mutilated by having the outer extremity 

 broken away. This attenuation is probably due in part, at least, 

 to the excessive wear this part of the coronal surface has been sub- 

 jected to while in use, and the vicissitudes to which they have been 

 exposed subsequent to their disengagement from the maxillary sup- 

 ports has resulted in disfiguring this part of the teeth so that the 

 form of the extreme antero-lateral border and outer extremity can 

 only be conjectured. However, it probably partook of the general 

 characteristics of the corresponding tooth of Stenopterodus and Del- 

 toptychius in truncation and definition of coronal and basal regions. 

 The far more uniformly massive posterior teeth of the mandible, 

 however, are often found quite entire. The position and consequent 

 severe usage suffered by the median teeth of the mandible by which 

 they have been defaced, as also their extremely simple form, may 

 have led to their being overlooked in many instances. 



Geological position and localities: The stratigraphical distribution 

 of the above species, as it is at present understood, is known to ex- 

 tend from the lower part of the Lower Coal Measures, well up into 

 the Upper division ; the vertical range in Kansas is known to include 

 above 1,500 feet of exclusively Upper Coal Measure strata ; LaSalle, 

 Springfield, Carlinville, Staunton, Belleville, Bloomington, Danville, 

 also, localities in Fulton and Vermilion counties, Illinois; Posey 

 county, Indiana; several localities in the Upper Coal Measures of 

 Iowa, Nebraska, Kansas and Missouri. 



OKTHOPLEUKODUS CONVEXUS, St. J. and W. 



PL XIII, Fig. 4, 5. 



Posterior teeth of mandible agreeing closely in outline with the 

 corresponding form of Orthopleurodus carbonarius, moderately arched 

 from within outwards and revolute, antero-lateral border moderately 

 curved sigmoidally, and occupied by a rather deep enamel belt be- 

 neath the sharp, anterior coronal angulation ; inner margin broadly 



13 



