EASTMAN : STRUCTURE AND RELATIONS OF MYLOSTOMA. 27 



of the lower jaw, against which they closed. Their posterior face is 

 smooth and slightly concave, as if for co-adaptation with the front margin 

 of the anterior pair of palato-pterygoid dental plates. These latter are 

 unfortunately missing in the type specimen, but as their allotted space 

 is accurately demarcated on all sides, it is easy to restore their outlines. 

 Thus, their posterior face must have abutted directly against the hinder 

 pair of palato-pterygoids, and their outer face have been conformable to 

 the external margin of the opposing lower dental plates ; this much, at 

 least, is certain. The inner face we should expect to be linear and 

 elongate, as in Mylostoma, in consequence of the close juxtaposition of 

 the two forward plates along the median line. That the corresponding 

 plates of Mylostoma were arranged in the manner indicated is too 

 evident to require demonstration ; it must be apparent to any one who 

 has ever handled the actual specimens, and applied their oral surfaces 

 against the lower dentition. 



The hinder pair of palato-pterygoid dental plates is excellently pre- 

 served, as will be seen from the illustrations given in Plate 2, Figures 

 16 and 17. These plates are elongate, of irregularly cruciform outline, 

 moderately thick, and develop on their oral surface a single, large, 

 rounded, centrally placed tubercle, which plays into a corresponding 

 depression of the lower dentition in the manner already described. A 

 peculiar feature of the plates in question is that the sinus which occurs 

 in about the middle of the posterior face shows vertical flutings and is 

 otherwise differentiated from the adjacent lateral edge of the plate. 

 It is difficult to imagine what purpose this excavation with fluted walls 

 could have served, unless it enclosed a passageway of some sort. The 

 opening is much larger than is needful to conduct nerves or blood- 

 vessels ; and besides, it communicates directly with the mouth cavity, as 

 is evident from the fact that the flutings are conterminous with both 

 the upper and lower surfaces of the plate. Now the only openings of 

 corresponding size and position that we are acquainted with in the 

 palate of any other fishes are the posterior nares, which are found only 

 in Dipnoans ; and the inference is by no means remote that we have to 

 do with these very structures in Dinomylostoma. 



Figure H is intended to show the relative positions of upper and 

 lower dental plates as determined for this genus, without, however, in- 

 cluding the anterior pair of palato-pterygoid elements. The vomerine 

 teeth are shown from their oral aspect, and in their natural position with 

 respect to each other, but removed further forward from where they 

 belong for the purpose of displaying the contour of the mandibular 



