VERTEBRATES. 289 



allied to Sybo-comjtressus of the npper Burlington limestone, it is readily 

 distinguished from that form by its more robust proportions, less com- 

 pressed crown, and less obliquely produced posterior expansion of the 

 base. The median cone presents the same lateral deflection, although 

 it is relatively less prominent, and it is possible the present form may 

 possess similar vertical rugte as appear in the crown of the above form. 

 Position and locality : Chester limestone ; Chester, Illinois. 



Genus THBI^ACODUS, St. J. and W. 



Teeth of small size. Base posteriorly produced in a long, sometimes 

 twisted, vertically flattened, or laterally compressed, clavate plate, 

 longer than wide, anterior face narrow, and abruptly beveled from the 

 basal line of the crown ; posterior extremity more or less obtusely 

 rounded; iuferior surface narrow, plane or faintly excavated; superior 

 surface gently couvex, concave anteroposterior^, or corresponding to 

 the curvature of the inferior surface. From the autero-superior 

 extremity of the base spring three more or less relatively stout, nearly 

 equal, trenchant, acutely pointed, recurved cusps, the exterior pair 

 divergent, the central one more or less vertical, slightly sigmoidally 

 curved, transverse section sublenticular, compressed in frout, rounded 

 behind, with simple cutting edges, aud more or less strongly costate in 

 either face. 



The generic peculiarities of the teeth here referred to, as contrasted 

 with Diplodus, Agass., with which they are most intimately allied, con- 

 sists in the relative more slender base, which lacks both the antero- 

 inferior protuberance and the postero-superior pad-like prominence 

 characteristic of the above genus. The coronal cusps are also nearly 

 equal in size, the cutting edges are destitute of crenulations, and their 

 anterior aud posterior faces more or less strongly ridged vertically. 

 The form of the trident-like cusps are very like that of Cladodus, being 

 slightly constricted basally in front aud faintly defined from the base 

 behind. 



As here defined, the genus includes the forms originally described by 

 Messrs. Xetvberey and Worthen, under the name Diplodus incurvus 

 and D. duplicatus, of the Keokuk limestone, which seem to be undis- 

 tinguishable from a form not uncommon in the upper Burlington lime- 

 stone. 



Thrinacodtjs nanus, St. J. and W. 



PI. V, Fig. ], 2. 



Teeth minute. Base thick, moderately produced posteriorly, or appa- 

 rently sub elliptical in outline, constricted or rounded below, and faintly 



-38 



