264 bulletin: museum of compakative zoology. 



theoretical reconstruction of Mylostoniid dentition, so that the adequacy 

 of his conclusions depends upon the trustworthiness of his initial con- 

 jecture, it will be seen that it is a matter of vital importance to ascer- 

 tain the true arrangement of these two pairs of palatal (or more 

 properly, palato-pterygoid) dental plates. 



Means for applying a critical test'to the two theoretical reconstructions 

 of parts that have been proposed has fortunately come to light during 

 the present year. This consists of a single very peculiar dental plate, 

 undoubtedly of Mylostomid nature which Dr. L. Hussakof describes 

 (Bull. Amer. Mus. Nat. Hist., May, 1909, 26, p. 269), under the name 

 of Dinognathus ferox n. gen., n. sp. In the opinion of the present writer, 

 this singular plate, which is of perfect bilateral symmetry and therefore 

 assignable to a median position in the mouth, cannot be interpreted 

 otherwise than as the fused pair of anterior palato-pterygoid dental 

 plates common to Mylostomids, using this term in the sense that it has 

 been heretofore employed for the type species of Mylostoma. That is 

 to say, under the term of anterior palato-pterygoid plates are to be 

 understood the narrow and elongate subtriangular elements which are 

 especially characterized by having straight vertical edges along their 

 mutual contact faces where the right- and left-hand elements were in 

 apposition with each other in the median line. We have merely to 

 suppose fusion to take place along these smooth and firmly adpressed 

 edges, in order to produce the symmetrical hippocrepiform outline ex- 

 hibited by the compound plate which is rightly made by Dr. Hussakof 

 the type of a new genus, Dinognathus. The sinuous lateral margins, 

 beveled, edges, expanded anterior and contracted posterior moieties of 

 this plate, together with the forwardly placed median tubercle of the 

 oral surface, are among the features which bring this plate into har- 

 monious relations with the two corresponding elements in M. variabile. 

 The general form and proportions of the plate, its relative thickness, 

 and, as noted by Dr. Hussakof, the density of its surface texture, all 

 testify to the correctness of the interpretation here advocated, and no 

 characters can be distinguished which militate against it. The size 

 of the single compound plate in Dinognathus does not greatly exceed 

 that of its two morphological equivalents in M. variabile, and agrees to 

 a nicety with the proportions indicated by the type mandible of 

 M. terrelli. Indeed, there are most excellent reasons for maintaining 

 a provisional association of the parts originally referred to two distinct 

 genera and species, namely, the above-mentioned type mandible of 

 M. terrelli, and the compound anterior palato-pterygoid dental plate 



