VERTEBRATES. 327 



with a faintly defined lateral carina in either face, but without visible 

 stiiatiou or other ornamentation, the anterior and posterior faces sharply 

 rounded ; at the base in either side, sometimes nearly median but gen- 

 erally slightly anterior to the median line, arise a pair of tumid, almost 

 rudimentary lateral denticles, which are not nearly as distinctly defined 

 from the median cusps as in the preceding and following forms herein 

 described. 



The tooth above described differs so markedly from the two other 

 species of the genus at present known, as scarcely to need farther com- 

 parison ; while, in its general features, it presents unmistakable generic 

 affinities with those more typical forms. 



Position and locality: Upper beds of the St. Louis limestone ; Alton, 

 Illinois. 



Periplectrodtts expansus, St. J. and W. 



PI. 8, Fig. 27, a, b, c. 



The single example of the present form which has come to our notice, 

 is distinguished by the great lateral expansion of the base, the gently 

 beveled upper edges being grooved in a manner similar to that observed 

 in P. Warreni, moderately arched antero posteriorly, the anterior extrem- 

 ity obtusely pointed, the posterior face of moderate depth, interrupted 

 by a shallow median sinus defined on either side by tumid protuberances 

 which converge into the acute posterolateral angles; the inferior sur- 

 face is somewhat deeply excavated, the concavity being transverse, the 

 lateral walls not so conspicuous as in P. Warreni. The coronal region 

 preserves three set of cusps, the anterior ones nearly obsolete from wear, 

 and which are characterized by the stout median cone, which is nearly 

 circular in transverse section or but slightly compressed antero-poste- 

 riorly, with extremely delicate lateral edges, and ornamented with 

 numerous delicate, sharp, widely spaced vertical costae; the lateral 

 cones, which are separated from the base of the principal cusp by only 

 slight intervening space, are exceedingly minute, rapidly tapering and 

 recurved, and ornamented similarly to the median cone. 



The unique example shows the two posterior set of coronal cusps in 

 perfect state of preservation as to abrasion, but the median cones are 

 broken off so that their entire length and shape is not shown. The form 

 bears closest resemblance to P. Warreni, described from the Upper 

 Burlington formation, from which, however, its extreme lateral expan- 

 sion and relatively shallow base readily distinguishes it. 



Position and locality : Lower fish-bed of the Chester formation ; 

 Chester, Illinois. 



