DEVONIAN FISHES OF IOWA 189 



fishes. As a necessary accompaniment of increase in size, the 

 cranial plates become more intimately fused in the adult, the 

 articulations between headshield and abdominal armor more 

 complicated, and various minor modifications are to be seen in 

 the dentition and arrangement of the body plates. Neverthe- 

 less, in spite of hypertrophic enlargement of all the parts, there 

 is everywhere a surprising conformity to the basal type of Coc- 

 costeus. This close correspondence has not been generally rec- 

 ognized, yet must be obvious to those who have actually com- 

 pared the different parts in well preserved specimens. 



The arrangement of dental elements in Dinichthys merits spe- 

 cial consideration. In the upper jaw there are always two pairs 

 of dental plates, whose structure clearly reveals their dermal 

 origin. It is denied by Dr. Bashford Dean that these plates, 

 which are known in common parlance as "premaxillaries" and 

 "shear-teeth", can be homologized with any structures within 

 the mouth of other fishes, hence he proposes to call them by the 

 non-committal names of "rostro-gnathals" and "orbito-gna- 

 thals" respectively, and employs the term "gnathal" for the 

 mandible. The same view is reiterated, with some further 

 changes in terminology, by Dr. L. Hussakof, in his "Studies 

 on the Arthrodira, ' ' 1906. 



According to the view adopted throughout this volume, the 

 anterior pair of upper dental elements in Dinichthys is to be 

 interpreted as vomerine, and the posterior as palato-pterygoid, 

 thus recognizing actual and definite homologies between them 

 and the like-named structures — which are also of dermal origin — 

 in modern Lung-fishes. The vomerine pair is situated close to 

 the median line, one on either side of the cranial plate corre- 

 sponding in part to the dermal mesethmoid in Neoceratodus ; 

 and the tumid basal expansion of each of these elements is re- 

 ceived into a slight concavity on the visceral surface of the 

 preorbital plate of the headshield. The exposed, or functional 

 portion of the vomerine teeth is cleft so as to form two vertical 

 prongs or "beaks" of unequal length, the shorter of which is 

 inwardly placed and abuts closely against its fellow of the oppo- 

 site side. The extremities of the vomerine teeth protruded very 

 slightly in advance of the mandibular beaks, which closed within 



