VEETEBEATES. 185 



the upper jaw. The latter is distinguished by its elongate subspat- 

 ulate shape, narrow lateral diameter and narrow alation, less rapidly 

 converging lateral borders and more oblique direction of the poste- 

 rior portion of the inner margin, relatively greater extent of the 

 articular edge of the antero-lateral border, (both borders being much 

 as in the same form of the representative Keokuk species, S. Icevis- 

 simus,) and the more uniform coronal contour, which does not pre- 

 sent to the same extent the strongly developed ridges characteristic 

 of the opposed mandibular teeth. In all, as also in the teeth sup- 

 posed to represent the mandibular median form, the coronal punc- 

 tation is essentially the same, being relatively fine, closely set, and 

 showing a tendency to elongation in the longitudinal direction of the 

 tooth, although very variable in this respect according to the condi- 

 tion of preservation of the coronal surface. 



Mandibular median teeth distinguished by their triangular outline, 

 rather strongly arched from within outward and strongly inrolled at 

 the outer extremity. Postero-lateral border nearly straight, defined 

 above by the rounded enamel fold, basal portion channeled and 

 beveled inferiorly to an obtuse edge ; antero-lateral border very ob- 

 lique, rapidly converging from the subacute inner angle to point of 

 iurollment forming an angle of 40 to 50 with the opposite side, 

 basal portion beneath the enamel fold somewhat channeled and bev- 

 eled, merging into the thickened edge that defines' this side of the 

 tooth; inner margin making nearly a right angle with the postero- 

 lateral border, gently arched and somewhat more sharply rounded near 

 the posterior angle, worn specimens inbeveled inferiorly. Crown 

 transversely nearly plane, slightly arched near the postero-lateral 

 border into the low, obscurely defined coronal prominence ; enamel sur- 

 face near inner margin marked by more or less distinct lines of 

 growth parallel with the inner margin, also faint longitudinal plicae ; 

 worn surface exhibiting the usual minute, closely arranged puncta- 

 tion characteristic of the associated forms of the species. Inferior 

 surface moderately excavated, smooth, conforming in a general way 

 with the contour of the coronal region. A small individual measures in 

 postero-lateral border across the inner margin 10.5 mm. ; length along 

 transverse diameter to point of inrollment 6.5 mm. A large-sized 

 tooth is respectively29 mm. and 21 mm. in breadth and length. 



Compared with the corresponding form of S. Icevissimus, of the 

 Keokuk limestone, the mandibular median teeth of the present 

 species are distinguished by their less oblique antero-lateral border 

 and spirally inrolled outer extremity, and smoother coronal surface. 



