VERTEBRATES. 123 



terminating in the more or less produced, acutely rounded posterior 

 extremity; inner margin making two broad arches, the larger in 

 rounding the base of the postero- coronal prominence, the lesser in 

 skirting the anterior prominence on the way to the obtuse anterior 

 angle, in ordinarily preserved teeth inbeveled inferiorly. Coronal 

 contour typical of the corresponding form of the genus, divided into 

 two principal regions occupied respectively by the posterior lobe and 

 the narrower anterior prominence. The posterior prominence cul- 

 minates about one-fourth the distance from the shallow sulcus 

 forming the demarkation between the coronal elevations, the 

 anterior acclivity rising quite steeply into the obtusely rounded 

 crest, whence the surface much more gently declines in a slight 

 concavity to the postero -lateral border, where the enamel forms a 

 narrow fold distinctly denned from the basal portions ; anterior 

 lobe more regularly and gently arched transversely, the longitudinal 

 axis lying near the median depression with a narrow steep slope 

 on that side, the wide, gentle anterior slope being interrupted by 

 more or less distinct revolving furrows, margined along the antero- 

 lateral border by a narrow plane belt denned from the anterior 

 prominence by a slight impressed line, the coronal enamel forming 

 a very narrow fold along the antero-lateral border. Greatest 

 breadth of medium-size tooth across inner margin 21 mm.; length 

 of antero-lateral border to inrolled outer margin 8 mm.; greatest 

 length along crest of posterior lobe 13 mm.; position of median 

 depression a little anterior of a line drawn through the centre of the 

 tooth. A large size specimen in the collection of Mr. Van Home, 

 from Alton, is quite one-third larger than that indicated in the 

 above measurements. 



Although the collections afford no positive evidence of the specific 

 identity of the dental elements belonging to the anterior extremity 

 of the upper jaw of the present species, there is, however, a well 

 represented form of teeth occurring in series precisely after the 

 Helodus-like teeth associated with the magnificent group of teeth 

 furnishing the basis for the description of the species Chitonodus 

 latus, (Leidy sp.) (Cocldijdus nobilis, N. and W.). More frequently 

 occurring as isolated teeth, examples of two or more teeth occupy- 

 ing their natural relative position in deltoid series have been ob- 

 tained, so that little doubt can be entertained as to their relations 

 with some Cochliodont, probably to the species herein mentioned. 

 Teeth occurring in series of two or more individuals, together form- 

 ing a deltoid figure, narrowing from within outward, and presenting 



