VERTEBRATES. 



207 



bordered exteriorly by a nearly straight, vertical face similarly 

 marked to the other vertical sides by vertical beaded rugosities. 

 This specimen, if it is correctly deciphered, has a nearly rectangu- 

 lar outline so far as relates to the external border and the anterior 

 and posterior margins, the angulation of the inner border giving the 

 tooth a pentangular figure. Taking into consideration the vertical 

 condition of the straight exterior border, which shows features con- 

 sonant with an articular face, this tooth certainly suggests a form 

 not hitherto recognized, and which is at the same time a true 

 Psammodus. It is possible that the specimen represents one side of 

 a median series of teeth comprising two ranges, the inner articular 

 borders alternating and the exterior borders joining the inner artic- 

 ular borders of teeth of the form of those described under P. 

 Springeri, &c. ? The specimen is strongly suggestive of such a com- 

 bination of dental elements, after the fashion sketched in the an- 

 nexed cut, Fig. a. The specific 

 identity of the " division bed " 

 tooth and that last referred to 

 from the main fish-bed of the 

 Upper Burlington, is only conjec- 

 turally inferred. The partially 

 shaded pentagonal plate on the 

 right of the double median series 

 gives a reduced outline of the 

 ff S thetiealdiagramofPsrtmmodMS tur ~ specimen here referred to. 



Geological position and locality: Fish-bed of the Upper Burlington- 

 Keokuk division beds ; DesMoines Co., Iowa. 



PSAMMODUS LOVIANUS, St. J. and W. 



PI. XIV, Fig. 7-9. 



Teeth attain medium size. Maxillary form subrhomboidal in out- 

 line, rather strongly arched from within outward. The margins 

 gently arched forward and parallel, the anterior margin apparently 

 somewhat strongly inbeveled, though really vertical to the edge of 

 the crown surface, posterior margin showing a belt of enamel ex- 

 tending perhaps one-fourth its depth, and well defined from the 

 slightly excavated base. Inner border making nearly a right-angle 

 with the margins, the truncated posterior angle reaching about one- 

 fourth the distance of the border, and making a rather strongly 

 marked angulation with the anterior portion of the articular face. 



