VERTEBRATES. 145 



DELTODUS LATIOB, St. J, and W. 



PL IX, Fig. 11-12. 



Mandibular posterior teeth moderately arched longitudinally and 

 rather strongly inrolled, in general outline and dimensions resemb- 

 ling the same form of Deltodus occidentalis with which they are 

 associated. Antero-lateral border slightly incurved, in length exceeded 

 by the breadth across the inner margin, enamel fold denned from 

 the basal border by a shallow groove ; general course of inner margin 

 slightly oblique to the anterior border, broadly arched round the 

 base of the coronal prominence with a slight concavity between the 

 median and the posterior angles; postero-lateral border gradually 

 converging toward the outer margin. The upper edge defined by a 

 rather thick fold of the coronal enamel. Coronal ridge most prom- 

 inent in the middle portion of its extent, depressed near the inner 

 margin, culminating in an obtusely rounded crest a little anterior 

 of the median line, anterior slope flat or slightly depressed, pos- 

 teriorly steeply inclined into the depression on that side, which is 

 bordered by the rather wide moderately upraised alate expansion. 

 Surface punctation very like that observed in D. occidentalis. Length 

 of a medium size tooth along the antero-lateral border to inrolled 

 outer margin 15 mm. ; breadth across inner margin 19 mm. 



The posterior form of the mandible alone is known, of which the 

 collections contain but few examples. These present, in the contour 

 of the crown and proportions of the coronal prominence and alate 

 lobe, characters which certainly offer striking contrast to those prev- 

 alent in Deltodus occidentalis, with whose remains they are asso- 

 ciated. The same observations are equally admissible when com- 

 pared with the same form of D. undulatus, N. and W., also of 

 the Keokuk, and D. spatulatus, N. and W., of the Upper Burlington 

 limestone. In the depressed anterior slope and the consequent 

 greater or less prominence of the coronal ridge, these teeth bear 

 some resemblance to the homologous form of Cochliodus; however, 

 the condition of the coronal fold of the antero-lateral border as also 

 the coronal contour leave no room for doubt as to their congeneric 

 relations with Deltodus. The paucity of examples and the absence 

 of identifiable individuals of the opposite jaw have occasioned much 

 hesitation in recognizing the specific distinctness of the teeth here 



alluded to; the materials illustrating D. spatulatus, D. occidentalis, 

 10 



