Eastman — Dijynoan Affinities of ArtJirodires. 143 



descendants of these latter have alone survived until the 

 present day. 



4. The primitive stock must have been autostylic, diphycer- 

 cal, without a secondary upper jaw and dentigerous dentary 

 elements, and with Uronemus- or Dipterus-like type of denti- 

 tion ; characters which do not permit ns to ascribe the ultimate 

 origin of Dipnoans to the Crossopterygii, but suggest rather a 

 descent from Pleuracanthus-Wke sharks. 



5. The recognition of Arthrodires as an order of Dipneusti 

 precludes their association with Ostracophores in any sense 

 whatever. The "Placodermata," as originally understood by 

 M'Coy and Pander, is therefore an unnatural assemblage, and 

 should be abandoned. 



EXPLANATION OF FIGURES. 



Fig. 1. Reconstruction of the upper dentition of Mylostoma variabile 

 Newb., from the Cleveland shale of Ohio. The small obtuse vomerine teeth 

 are succeeded behind by two pairs of palato-pterygoid dental plates, as in the 

 young of Neoceratodus. The dotted lines are intended to indicate the posi- 

 tion of the supporting palato-pterygoid cartilage. x}4- 



Fig. 2. Mandibles of Dinomylostoma beecheri(MS.), from the Portage beds 

 of Mt. Morris, N. Y. Both the right (a) and left (b) mandibles are shown 

 from the lateral, external aspect. The posterior portion of the splenial in b 

 is seen to have the compressed articular cartilage attached to its outer side. 



Fig. 3. Restoration of the cranial shield in Dinichthys pustulosus Eastm. 

 from the Middle Devonian of Iowa. C, central ; EO, external occipital : M, 

 marginal ; MO, median occipital ; P, pineal ; PO, preorbital ; PtO, post- 

 orbital ; R, rostral or mesethmoid. Sensory canals represented by double 

 dotted lines, x }<£. 



Fig. 4. Cranial roof of the recent Neoceratodus forsteri Krefft, drawn as 

 if flattened out, and dermal plates lettered to correspond with those of Din- 

 ichthys. The anterior median plate is commonly termed mesethmoid. x %. 



Harvard University, 



Cambridge, Mass. 



