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BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



The complete dental apparatus, portions of which have been referred, 

 according to our view, to two distinct " species " and even genera, should 

 henceforth bear the designation of Dinognathus terrelli (Newberry). Not 

 only does the solitary mandible upon which this species was originally es- 

 tablished (now the property of the Museum of Comparative Zoology) differ 

 in appreciable respects from the type of M. variabiles but the more com- 

 pactly formed upper dental pavement — as witnessed by the type of 

 Hussakof's so-called Dinognathus ferox — justifies the recognition of 

 generic differences between it and other known Mylostomids. Figures 



Fig. 3. Proposed reconstructiou of Mylostoniid type of dentition, based upon 

 the originals shown in Figure 1. One of these elements together with the 

 containing slab is preserved in the Museum of Comparative Zoology (Cat. No. 

 1490) ; the remaining elements and opposite half of the counterpart in the Ameri- 

 can Museum at New York (Cat. No. 7526). x \. 



2 and 3 aflFord means for a comparison of the restored palatal dentition 

 of Dinognathus and Mylostoma. 



The manner of application of a critical test to our hypothesis will be 

 apparent from a study of the above illustrations. For it will be observed 

 first of all that there is no possible doubt that the compound crushing 

 plate of the so-called D. ferox type must have occupied a median position 

 in the dental pavement of which it formed a part. And in order to 

 have operated effectively against the oral surface opposed to it by the two 



