VEKTEBRATES. 181 



into the border of the narrow alate expansion. Surface structure 

 minutely and densely punctate ; in the less abraded areas, especially 

 toward the inner margin, the punctse are arranged in more or less 

 regular parallel transverse lines, conforming in direction to the course 

 of the inner margin. This latter feature, however, is seldom ob- 

 served to extend over any considerable area of the coronal region, 

 which oftener shows the ordinary punctate structure with only faint 

 traces of the transverse striation alluded to. Length of a medium- 

 size tooth along the antero-lateral border, 10 mm.; breadlh at inner 

 margin, 4.5 mm.; elevation of antero-lateral border, 1.5 mm. 



The collections afford but few examples of the present species, 

 amongst which occur no examples of the bioad alate form attribut- 

 able to the terminal form of the mandible. Compared with the 

 Chester species, Xystrodus verus, it is distinguished by the propor- 

 tionately greater breadth, generally smaller size, and the less' pre- 

 valent and distinct transverse disposition of the punctae of the coronal 

 surface. In proportions and general outline, the species bears strik- 

 ing resemblance to the corresponding form of X. striatus, Agass., of 

 the Mountain limestone, Armagh, but it differs in being smaller and 

 in the less marked transverse striation. 



Geological position and localities: St. Louis formation; St. Louis 

 (Missouri) ; Alton, and Monroe county (Illinois), and Pella (Iowa). 



XYSTRODUS VERUS, ST. J. AND W. 



PI. VIII, Fig. 6, 7. 



Maxillary posterior teeth cuneiform, moderately arched from with- 

 in outward. Antero-lateral border with slight sigmoidal curvature, 

 abruptly beveled from the coronal angulation, which is obtuse in 

 the region of the inner margin and more acute toward the outer 

 extremity, probably in consequence of the wearing down of the tri- 

 turating surface, articular border, comprising three-fourths the extent 

 of this side, the upper portion enveloped in the well-defined enamel 

 fold, basal border terminating inferiorly in the thin compressed rim ; 

 inner margin broadly rounded at the base of the coronal prominence 

 and gently deflected backward to the posterior angle of the moderately 

 produced alation ; postero-lateral border slightly curved, very gradu- 

 ally converging toward outer extremity, coronal enamel forming a 

 narrow fold, inbeveled and well defined from the abrupt downward 

 and slightly outward projecting basal portion. Coronal surface oc- 

 cupied by a broad anterior prominence, slightly arched transverse- 



