VEETEBEATES. 



amounts to a slight convexity in the long diameter of the tooth, the~ 

 outer border heing well denned hy a narrow ridge, separated from 

 the convex inner area by a shallow, longitudinal depression. The 

 anterior margin is gently arched, with a slight concavity near the 

 inner angle and rounded into the outer border at the opposite angle, 

 the inbeveled basal portion meeting in a sharp angle with the coronal 

 surface ; posterior margin describing approximately the same curv- 

 ature, the crown defined by a sharp angle, the slightly-inbeveled 

 basal portion channeled, the upper portion enveloped in a glassy, 

 enamel layer; outer border gently arched, terminating posteriorly 

 in a more or less produced spur, the narrow coronal ridge rounded 

 to and well defined from the basal rim, which latter is not suf- 

 ficiently well-preserved to show its actual outline and magnitude. 

 The inner border is nearly straight, with a slightly-oblique course 

 forward and outward, and faint angulation near the anterior angle, 

 the crown making a sharp angle with the channeled basal wall, and 

 margined by a slight-impressed line, forming a narrow border en- 

 circling the contiguous sides of the crown. The inferior surface, in 

 the perfect state, shows irregular striato-punctation in the slightly- 

 concave, longitudinal direction, with a slight elevation about one- 

 third the distance from the inner border, where the tooth attains its 

 maximum depth. The coronal surface is uniformly minutely punc- 

 tate, plainly exhibiting the superficial structural features common to 

 the congeneric forms. Transverse diameter across the posterior mar- 

 gin, 29 mm. ; length along inner articular border, 17 mm. ; greatest 

 depth of tooth, 6.5 mm. 



Worn examples of the above described form show a greater trans- 

 verse convexity of the coronal surface, while the edges are rounded, 

 obliterating the narrow, impressed border mentioned above ; it is not 

 improbable also the outer ridge may become nearly obsolete in sim- 

 ilarly preserved specimens. The homologous form of the St. Louis 

 species Psammodus plenus presents a striking contrast to the pres- 

 ent teeth, being proportionately longer, with less inbeveled posterior 

 and anterior walls, and greater lateral convexity of the coronal sur- 

 face. Indeed, the nearest allied species at present known is that 

 from the Upper Burlington, P. Springcri, the corresponding form of 

 which differs in the directly opposite direction of the obliquity of 

 the inner articular border, greater length, and more produced pos- 

 terior angle of the outer border. 



In accordance with the presumed homological relations of the va- 

 rious forms of teeth ascribed to the genus, the form pertaining to 



