VERTEBRATES. 335 



cusps, which regularly increase in size posteriorly and as regular in the 

 order of their succession, the posterior one produced nearly its entire 

 length beyond the basal border ; the crown is enveloped in a polished 

 layer of enamel, the cusps are apparently destitute of ornamentation. 



A solitary specimen is all we possess from which to recognize the 

 above described form, but which is amply sufficient to show its distin- 

 guishing features in comparison with the previously noticed forms from 

 the Upper Burlington formation. Of the latter, tbat described under 

 term S. simplex most intimately approaches the form here indicated. 

 But the present tooth is distinguished by its more compact, robust build, 

 and especially by the more produced lateral borders of the crown, and 

 proportionately deeper base. 



Position and locality ; Lower fish-bed of the Chester formation ; Ches- 

 ter, Illinois. 



Gtejtus LEIOD US, St. J. and W. 



Teeth of medium and small size, in general form like Orodus. Crown 

 more or less arched laterally and vertically, basal margins constricted 

 and sharply defined from the base, culminating in a more or less pro- 

 duced apex with obscurely defined lateral crests, convex in either face, 

 the anterior face generally produced or buttressed beneath the median 

 cone, and both faces occupied by comparatively faint, simple vertical 

 sulci, which sometimes extend to the crest, producing obscure secondary 

 prominences; surfaces generally smooth, polished, punctate, and some- 

 times, especially in immature teeth, verrucose or reticulately roughened 

 in the basal margins. Base, as in Orodus, relatively deep, gently con 

 vex behind, concave in front, with a more or less prominent shoulder 

 above parallel with the inferior coronal margin, below obliquely beveled 

 to the inferior edge. 



The teeth here referred to are closely allied to Orodus, from which they 

 are mainly distinguished by the comparative smoothness of the coronal 

 surfaces, which are simply and generally obscurely lobed, or quite plain, 

 instead of being corrugated. And the apparent diversity in the form 

 of the teeth from different parts of the jaws, indicates a much more 

 varied dentition than obtains in Orodus, in this particular approaching 

 the modern Gestracion. The following are the sole species as yet indi- 

 cated. 



