THE LOWER EM BAR OF WYOMING 643 



tions. The crown is smooth but shows, a tendency to become 

 convoluted in old age; the surface is finely and evenly punctate. 

 Convex lateral margins terminate in a rounded edge with incipient 

 crenulations in old forms. The concave edge is vertical and has 

 evenly spaced crenulations on the lower half. The smaller teeth 

 are more strongly arched in both directions than the larger. 



This species differs from Helodus politus in having the crenula- 

 tions on the sides of the teeth evenly spaced, while they are generally 

 unevenly spaced in the latter form; they are regular in size in this 

 form, but irregular in H. politus; of almost uniform width from 

 top to bottom, but thicker at the bottom in H. politus; they are 

 longer than in H. politus. The crenulations are generally on a 

 retreating edge, while those of H. politus are on a protruding edge. 

 H. politus teeth have a tendency to form a marked projection, 

 almost a boss, in the middle of the tooth, while this is only rarely 

 present in H. subpolitus. H. subpolitus becomes convoluted in old 

 age; this is not true of H. politus. 



These teeth are the most abundant in the Embar formation but 

 their vertical range was not found to be more than 4 feet. More 

 than 300 specimens are in the paleontological collection of the 

 University of Missouri. 



Helodus rugosus N. and W. (PL II, Fig. 20) 

 1870. Helodus rugosus N. and W., Geol. Surv. III., IV, 359, PL II, Fig. 10. 

 1889. Helodus rugosus Woodward, Catalogue of Fossil Fishes in the British 



Museum, Part I, p. 227. 

 1903. Helodus rugosus Eastman, Bull. Mus. Comp. Zool., XXXIX, 182-83, 

 PL 2, Fig. 14. 



One Embar specimen agrees with the original description of 

 this form, though the peculiar surface marking is indistinct. 

 Crassidonta stuckenbergi gen. and sp. new (PL I, Figs. 17-27) 



The main part of the tooth is a short, symmetrical, rounded ridge 

 with its long axis in the line of the long axis of the tooth, and 

 flanked on either side by narrow, thickened, winglike extensions. 

 The ridge makes up two-thirds of the height of the tooth and one- 

 half of the width. Its sides are steep and the top is gently rounded 

 transversely and arched longitudinally. Near one end the longi- 

 tudinal arching is sharply increased. The crowns of the smaller 



