EASTMAN: MYLOSTOMID DENTITION. 221 



tation of the posterior palatal plates is determined with absolute accu- 

 racy. Another point confirmatory of the new arrangement is that in 

 the nearly complete example of Mylostoma (Fig. .4), the posterior pair of 

 palatal plates occurs symmetrically oriented with reference to the median 

 line. From this it is evident that the only one of the four palatal 

 plates whose position has been disturbed, aside from lateral displacement, 

 is the right anterior. 



The present reconstruction of the palatal dentition of Mylostoma 

 thus provides a consistent explanation of all observed facts, and is at 

 variance with none of them. It is free from theoretical objections, is in 

 harmony with analogy, both with Dinichthys and Ceratodonts, and is 

 applicable to all specimens thus far discovered, whether found in the 

 detached or associated condition. Its correctness may therefore be 

 regarded as definitely proved. Up to the present point the discussion 

 has been purposely restricted to the palatal and mandibular dental 

 plates. 



Concerning those structures interpreted by Newberry as "preman- 

 dibular," and by the present writer in earlier papers as " vomerine "teeth, 

 it is now necessary to speak more particularly. I purpose showing 

 that the three known specimens which have received this designation 

 do not belong to the type species of Mylostoma, but to a smaller, very 

 distinct form, presently to be described under the name M, newberryi. 

 Moreover, the specimens in question are no longer interpreted as 

 vomerine, but as mandibular plates which have become accidentally 

 dissociated from their supporting splenials, or from the greater part of 

 these bones. The presence of vomerine teeth in Mylostoma is therefore 

 not yet demonstrable by any positive evidence that has come to light, 

 although their potential occurrence is to be inferred from analogy with 

 Dinomylostoma and Coccosteans generally. 



Indications of a New Species of Mylostoma. — As already stated, 

 Newberry was of the opinion that the lower dentition of Mylostoma 

 consisted of at least two pairs of dental plates, mandibular and " preman- 

 dibular," opposed to which in the upper jaw was a " tesselated pavement 

 consisting of many pairs of plates." Bashford Dean was enabled to 

 show that the upper pavement dentition was made up of two pairs 

 of plates only, against which functioned the single mandibular pair. In 

 the nearly complete example of M. variabile studied by this author no 

 trace was observed of yet another fourth pair of dental structures, cor- 

 responding to those named premandibular by Newberry, and vomerine 

 by the present writer. The originals of Newberry's figures do not enter 



