PLATE XVIII. 



PAGE, 



PSAMMODUS C.&LATUS, St. J. and W 217" 



Fig. 1 a. Maxillary tooth, view of triturating surface, inner articular border restored 

 in outline; 1 b. do., exterior border; 1 c. do., posterior margin; 1 d. do., out- 

 line from anterior margin; 1 e. do., longitudinal profile section. St. Louis; 

 Pella, Iowa. Mr. Van Home. 



PYAMMODUS CRASSIDENS, St. J. and W 218 



Fig. 2 a. A small tooth, showing the coronal surface worn smooth, view from above; 

 2 b. do., inner articular border; 2 c. do., exterior border; 2 d. do., posterior 

 margin; 2 e. do., anterior margin. St. Louis; Pella, Iowa. Mr. St. John. 



Fig. 3 a. A relatively shorter tooth of the same form, from above, preserving traces of 

 the rugose coronal surface; 3 b. do., articular border; 3c. do., exterior bor- 

 der; 3 d. do., articular border; 3 e. do., posterior margin; 3 /. do., inferior 

 surface; the dense external layer exfoliated. Original of "Ps, rugusus," 111. 

 Geol. Surv. II, PI. XI, f, 3, 3. St. Louis; Alton. 111. Professor Worthen. 



Fig. 4 a. A more elongate example of the same form, the triturating surface preserv- 

 ing traces of the transverse rugae; 4 b. do., articular border; 4 c. do., exterior 

 border; 4 d. do., posterior margin. St. Louis; Alton. Mr. Butters. 



Fig. 5 a. A large entire example of the same form, the coronal region worn smooth, 

 and showing the prominently developed basal rim of the exterior border; 5 

 b, do., articular border; 5 c. do., from the exterior border; 5 d. do.,posterior 

 margin; 5 e. do., anterior margin. St. Louis; Alton. Mr. Van Home. 



Fig. 6 a. An immature large tooth of the same form, the inner half of the coronal sur- 

 face presenting beautiful rugose markings; 6 b. do., articular border. St. 

 Louis; Alton. Mr. Butters. 



