258 PROCEEDINGS OF THE NATIONAL MUSEUM. vol, 52. 



accordance with Jaekel's proposed association of parts, already re- 

 ferred to.^ One such tooth in the National Museum collection is cata- 

 logued as No. 8103, and represented of the natural size on plate 8, 

 figure 6. 



Formation and locality. — St. Louis limestone (Mississippian) ; near 

 Alton, Illinois (from the Hamhach collection). 



Family PSAMMODONTIDAE De Koninck. 



Genus PSAMMODUS Agassiz. 



The teeth in this genus are quadrate, more or less elongated, rarely 

 nearly square; root much thicker than the crown, and readily de- 

 tached from the latter ; coronal surface generally marked by transverse 

 rugae, complete dentition consisting of from two to four longitudinal 

 rows of pavement teeth in different species, as inferred from their 

 form and marks of mutual contact. 



PSAMMODUS PLENUS St. John and Worthen. 



Plate 7, fig. 8. 



Psammodus plenus St. John and Worthen, Pa!. Illinois, vol. 7, 1883, p. 213, 



pi. 16, figs. 1-4; pi. 17, figs. 1-4. 

 Psammodus glyptus Newberry (errore), Monogr. U. S. Geol. Surv., vol. 16, 1889, 

 p. 210, pi. 19, figs. 7, 8. 



The teeth of this species attain large size, and, according to the 

 original authors, it is possible to distinguish between those belonging 

 to upper and lower dental pavements. The form supposed by them 

 to pertain to the upper jaw is described as " subrhomboidal, or of a 

 laterally elongate-trapezoidal outlme, gently arched antero-poste- 

 riorly." The supposed mandibular teeth "are distmguished by their 

 sub quadrangular outline and relatively narrower transverse diameter 

 compared to the length." 



According to the interpretation of the authors just quoted the teeth 

 "were ranged in double rows upon the jaws," in the same manner as 

 mdicated by them in the ease of P. springeri, and by Newberry in 

 the case of his so-caUed Arcliaeohatis gigas, which latter can scarcely 

 be maintained as a distinct genus. There exist, however, certain 

 teeth of P. plenus which from their bilateral symmetry are assign- 

 able to a median position in the mouth, either above or below; and 

 these were evidently bordered on either side by a row of lateral teeth. 

 Two such teeth which we interpret as indicative of a median unpaired 

 row have already been figured by Newberry under the erroneous 

 designation of P. glyptus;^ the original specimens are now preserved 

 in the American Museum of Natural History, and have been compared 

 by the writer with other teeth of the same species. The markings 



1 Zcitschr. Deutsch. Geol. Ges., vol. 51, 1899, p. 280. 



2 Monogr. U. S. Geol. Surv., vol. 16, 1889, pi. 19, figs. 7, 8. 



