262 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Whether Mylostoina proper possessed the basic number of three pairs of 

 plates in the upper jaw has not yet been determined by positive evidence. 

 Certain detached triangular elements that were provisionally interpreted 

 bj Newberry as " premandibular" plates belonging to the type species 

 of Mylostoma, and afterwards by the present writer as vomerine teeth 

 referable to the same form, have since been assigned to the lower jaw of 

 a distinct species, named M. neicherryi (Bull. M. C. Z., 1907, 50, p. 224). 

 Reasoning from analogy, however, the presence in Mylostomids of vome- 

 rine teeth, which are the morphological equivalents of the so-called 

 " premaxillary " (in reality vomerine) teeth of Dinichthys, becomes a 

 necessary postulate. 



Thus far decisive evidence has been lacking which shall enable us to 

 establish the homologies between the several components of Mylostomid 

 and Dinichthyid deutiLiou. For, if we accept Dean's interpretation and 

 regard one of the pairs of pavement plates in Mylostoma as the modified 

 " premaxillary " (i. e., vomerine) teeth of Dinichthys, we shall perforce 

 have to deny the possible occurrence of a third pair of plates in this 

 genus, corresponding to the most anterior of the trio that have been 

 definitely proved to belong to Dinomylostoma. Taking this latter fact, 

 however, for our point of departure, and noting tlie obvious correspond- 

 ence between the similarly placed vomerine teeth in Dinomylostoma 

 and Dinichthys, it follows that the two pairs of pavement plates in 

 INIylostomids generally must together correspond to the single large 

 sectorial plate or " shear-tooth " of Dinichthys. The latter, therefore, 

 according to this view, is to be regarded as a morphologically compound 

 plate, made up of the fusion of two elements that remain separate in 

 more primitive forms ; and as a corollary to this theory, the discovery 

 at some future time of true vomerine teeth in Mylostoma maj' be confi- 

 dently expected. 



Up to the present time material has been lacking which shall permit 

 a complete verification of either of the above alternative hypotheses. 

 Neither has the precise arrangement of the two pairs of Mylostomid 

 dental plates been demonstrated by the discovery of the undisturbed 

 upper dental pavement belonging to a single individual, or even such 

 portions of the pavement as can clearly be proved to have been pre- 

 served in normal position. In the single nearly complete individual 

 studied by Dean the two plates belonging to the right-hand side of the 

 palatal pavement are indeed juxtaposed in the matrix of the containing 

 slab. (Fig. 1.) 



Although at first sight these tritoral plates might be supposed to 



