EASTMAN : STRUCTURE AND RELATIONS OF MYLOSTOMA. 25 



rather obtuse symphysial beak, but slightly elevated above the broad, 

 flat, deeply excavated functional surface. The latter displays a single 

 inconspicuous tubercle close to the external margin, situated about 

 midway its length, and its posterior termination is marked by a still 

 larger tubercle, rather elongate, and placed externally like the first. 

 This posterior prominence fits snugly against the single large rounded 

 boss of the opposing palato-pterygoid dental plate, the adjustment being 

 such as to orient the hinder pair of upper dental plates with absolute 

 precision. It is fortunate that we have this topographic control, since it 

 serves as a check upon any theoretical reconstructions of the upper 

 dental plates of Mylostoma that might be attempted. 



The lower dental plate proper, or that portion of the mandible which 



Fig. F. — Portion of left suborbital of Dinomylostoma beecheri Eastman, from the 

 Portage of Mt. Morris, N. Y. External aspect, x £. 



corresponds to the lower dental plate in related forms, is so intimately 

 united with the supporting splenial that the two might be said, so far as 

 appearances go, to form an integral piece. The same, however, might 

 be said of modern Dipneumon genera, and if one is really in doubt 

 whether two separate entities are concerned, that doubt is dispelled by 

 the most cursory examination of the mandible in Mylostoma, where we 

 find a veritable pharos for indicating Dipnoan affinities. The splenial 

 of Dinomylostoma is developed as a long, slender posteriorly rounded 

 shaft, much resembling that of Dinichthys, but relatively a little deeper. 

 A fact of great interest is that both of these elements, the right and 

 the left, are preserved in natural association with the articular cartilage, 

 and this, although considerably compressed by mechanical processes, is 

 seen to form a hinge for attachment with the suspensorial cartilage of 



