VEBTEBKATES. 159 



mit. The antero-lateral and postero-lateral borders are marked as 

 in Deltodus. The coronal fold of the articular border, perhaps, more 

 rounded and relatively narrower than that of the postero-lateral 

 border. 



Teeth, identified with the median form of the mandible, are char- 

 acterized by their thin, oblique, trapezoidal figure, great length of 

 the posterior, as compared with that of the anterior, articular bor- 

 der, and its strong outward and forward obliquity. The basal rim 

 and enamel fold corresponding in co-adaptation with the opposed bor- 

 der of the posterior or terminal tooth, and essentially like the thin- 

 ner and very short antero-lateral border ; inner margin very oblique 

 from the subacute posterior angle forward to the obtuse anterior 

 angle ; coronal surface comparatively plain, or culminating in a low 

 ridge near the posterior side. 



The maxillary posterior teeth are characterized by their elongate, 

 wedge-shaped outline, narrow inner margin, and the narrow posterior 

 alation. The postero-lateral or oblique border shows the basal por- 

 tion inbeveled, channeled, and defined above by a distinct, inbeveled 

 enamel fold; the opposite articular border is relatively short and 

 mostly enveloped in the enamel fold, beneath which projects the 

 narrow basal rim ; the inner margin, as in the corresponding form 

 of Deltodus, is very oblique in its course from the very obtuse angle 

 of the articular border backward to the base of the coronal ridge, 

 where it is abruptly deflected at right angles, thence to the posterior 

 extremity. The crown presents a simple prominence or ridge along 

 the anterior side, with abrupt slope to the articular border, and 

 behind, descending into a shallow depression, from which rises the 

 narrow wing expansion. 



The superficial punctation is very like that observed in Deltodus, 

 with similar specific modifications. 



There exists so intimate relationship between the forms here 

 alluded to and typical Deltodus that we have hesitated in recog- 

 nizing for them more than a subgeneric distinctness. But taking 

 into consideration the community of characteristics by which they 

 are distinguished, and which equally serve to distinguish them from 

 Deltodus, this discrimination seems to be founded in tangible super- 

 ficial characters that fail not to impress the observer with their im- 

 portance. The greater the familiarity with the various examples 

 embraced under this group, the differentiation, or tendency thereto, 

 of the median ridge of the anterior coronal prominence further 

 widely departs from typical Deltodus, while at the same time it 



