VERTEBRATES. 191 



Mandibular median or second teeth characterized by their trian- 

 gular outline, rather strong inrollment of the outer extremity, 

 straight postero-lateral border which is similarly enameled to the 

 antero-lateral border of last above described posterior dental plates, 

 antero-lateral border rapidly and irregularly converging from the 

 subacute angle of the broad, slightly arched inner margin. Coronal 

 surface forming a broad, low arch, or nearly plane transversely. 



Although in every case detached and separated from their natural 

 position upon the jaws, there is strong inferential evidence of the 

 dental elements described above having constituted parts of the den- 

 tition of one and the same fish, as will appear more at length 

 under the head of specific descriptions. The genus as here consti- 

 tuted does some violence to the interpretation of the affinities of 

 the teeth attributed by authors to other genera, but the facts are 

 believed to justify no other conclusion, while in the typical species, 

 at least, there can hardly exist a doubt as to the specific identity of 

 the widely dissimilarly shaped teeth alluded to in the foregoing 

 generic diagnosis. 



The genus as now understood includes, besides the American 

 forms hereinafter described, possibly a form of teeth from the Car- 

 boniferous limestone of Bristol (England), which has been referred 

 to Tomodus, Agass. All the specimens of maxillary teeth are muti- 

 lated by having their exceedingly thin anterior portion broken away ; 

 and this fact naturally suggests the possible occurrence of corres- 

 ponding dental plates in European collections however, in so frag- 

 mentary a state as not to have attracted attention to their probable 

 relationship with the more robust mandibular teeth. 



Compared with Sandalodus, N. and W., the following distinguishing 

 features are especially noticeable : The posterior form of the man- 

 dible is abruptly broken down in a prominent mural face along the 

 antero-lateral border, which is enveloped in a widish belt of the 

 coronal enamel, and the anterior position of the transversely, nearly 

 plane, well-defined coronal prominence, in all which there is striking 

 contrast with the very restricted antero-lateral articular border of 

 typical homologues of Sandalodus. In the posterior teeth of the 

 upper jaw, the coronal prominence, instead of being situated for- 

 ward of the median line, as in Sandalodus, rises close along the 

 postero-lateral border, the wide coronal declivity being transferred 

 to the opposite side of the tooth. The median teeth of the man- 

 dible, representatives of which belonging to either generic group 

 are exceedingly rare, present essentially the same characteristics 



