VERTEBRATES. 109 



3.5 m m., and length along the straight postero-lateral border about 

 2mm. Trapezoidal in outline, it is relatively broader than the 

 Upper Burlington teeth of the same form ; the postero-lateral border 

 shows a slight forward obliquity in its outward course, basal portion 

 exceedingly attenuated ; antero-lateral borders converging toward 

 point of inrollment at an angle of about 30 with the opposite 

 border, too imperfect to show the character of the basal rim be- 

 neath the narrow, enamel fold : inner margin very gently arched 

 from the anterior angle, and abruptly rounded to the obscure, pos- 

 tero-inner angle. Crown presenting a broad, faintly-depressed slope, 

 culminating in a low ridge closely bordering the postero-lateral side, 

 to which it is abruptly sloped and merged into the distinctly-defined, 

 narrow, marginal fold. Coronal punctation relatively coarse and closely 

 arranged, very similar to that observed in the mandibular and max- 

 illary posterior teeth. 



The specific relations of the above described form are, of course, 

 only inferentially determined. It might appear to be an anomalous 

 state of things, especially in view of the fact that the mandibular 

 and maxillary posterior forms of the species to which it is pro- 

 visionally attributed occur in comparative numbers, the latter form 

 should be represented by a unique example. But the small size of 

 the teeth, and their thin, scale-like fragility, may in part, at least, 

 account for their rare preservation and infrequency in collections. 



Geological 2Josition and localities : St. Louis limestone ; Alton and 

 Monroe county, (Illinois) ; St. Louis, (Missouri), and Pella, (Iowa). 



GENUS CHITO]S"ODUS, St. J. and W. 



Teeth representing the various forms characteristic of the genera 

 of Cochliodonts. 



Mandibular posterior teeth trapezoidal in outline, strongly to mod- 

 erately arched in the direction of inrollment. Antero-lateral border 

 generally but slightly oblique to a line projected between the ante- 

 rior and posterior angles of the inner margin, basal portion chan- 

 neled, continued downward and outward into a thin marginal rim, 

 bordered above by the narrow, inbeveled fold of coronal enamel; 

 postero-lateral border converging toward outer side or point of inroll- 

 ment at a moderate angle, basal portion expanded into a thin rim 

 projecting a greater or less distance beyond the coronal border and 

 terminating in the more or less produced spur of the posterior 

 angle, in front similarly channeled and defined above by the rounded 



