VERTEBRATES. 



specifically. The mandibular posterior, teeth are proportionately 

 shorter from within outward and wider across the inner margin, and 

 the median-lobe perhaps less well defined from the narrow anterior 

 belt; while the opposed tooth of the upper jaw has the secondary 

 lobe relatively much more strongly developed and the coronal prom- 

 inence occupying a greater portion of the crown surface. 



The state of preservation of these diminutive teeth, especially in 

 the instance of those pertaining to the mandible, is extraordinary ^ 

 But of the maxillary teeth the collections afford few examples, and 

 these are without exception mutilated at the antero-lateral border, 

 in consequence of the extreme although notMisproportionate attenu- 

 ation they undergo in that region. 



Geological position and localities : St. Louis limestone ; Alton and 

 Monroe county, 111. ; Pella, Iowa ; St. Louis, Mo. 



DELTOPTYCHIUS NITIDUS, (Leidy, sp.) 



PL V, Fig. 16. 

 Cochliodns nitidus, Leidy, 1856. Trans. Am. Phil. Soc., xi, p. 87. PI. v, f. 2. 



The present species was originally described by Dr. Leidy, from a 

 large-sized specimen of the posterior tooth of the left ramus of 

 the lower jaw, which was obtained at Chester, in this State. The 

 teeth are massive, and strongly built, triangular in outline, pos- 

 tero-lateral border rapidly converging toward the outer extremity, 

 inner margin broadly curved round the base of the median prom- 

 inence, antero-lateral border nearly straight, relatively short, the 

 enamel fold extending well down over the abrupt face. The basal 

 portion along the postero-lateral border, in a mature individual like 

 the type noticed by Dr. Leidy, extends in a conspicuous rim beyond 

 the well-defined enamel fold, terminating in a strong, bony spur, 

 posteriorly, and reaching the point of strong inrollment, where it pre- 

 sents the usual channeled condition. The crown is strongly arched 

 from within outward, median lobe embracing about half the lateral 

 diameter of the crown, usually prominently arched transversely, a 

 slight angulation defining it from the narrow anterior belt, the slope 

 behind descending into the posterior depression, which is bordered 

 by the relatively narrow alation. A medium-sized tooth measures, 

 along the postero-lateral border from the posterior angle to point 

 of inrollment, 12 mm. ; length along antero-lateral border, 7.5 

 mm. ; breadth across inner margin, 10 mm. 



The collections contain examples of maxillary posterior teeth, from 

 the same deposits and localities, associated with the above described 



