EASTMAN: THE DINICHTHYIDS. 31 



of parts as knorrn in Dinichthys does not hold true by any means for 

 other genera (Trachosteiis, Mylostoma, etc.) belonging to the same fam- 

 ily ; and this fact admonishes us not to press hypothetical correlations 

 too far, even within specific limits. Caution is enjoined in this particu- 

 lar case by yet another consideration. From the same locality and 

 formation, Mr. Mixer has obtained a pair of mandibles associated with 

 fragmentary Dinichthyid plates. The condition of these remains does 

 not warrant a precise specific determination, but their affinities are 

 probably with D. minor. The length of each ramus is about 17 cm., 

 and the maximum height 5 cm. Either, therefore, these remains and 

 the plastron represent together but a single species {D. 1 minor), or 

 we have evidence of two medium-sized species (Z). ] minor and D. % new- 

 herryi) in the Portage Shale. 



Under these circumstances it is apparent that a positive identification 

 of the species is impossible. For the sake of convenience, we might 

 follow Dr. Clarke's example, and refer all the detached plates occurring 

 in the Genesee Shales to D. newberryi, and all those from Portage Shale 

 to D. minor. But there is no reason for supposing that each of these 

 horizons contains but a solitary species ; the indications point rather to 

 the presence of more than one species in both horizons. And there is 

 no reason why the doctrine of correlation of parts should not be applied 

 to all the species of Dinichthys until experience has shown it to be in- 

 valid for some of them. Provisionally, therefore, we are in favor of refer- 

 ring the Portage plastron to the species with which it most closely agrees 

 in measurement and geological horizon, that is to say, with D. newberryi. 

 On the other hand, the Portage mandibles that have just been men- 

 tioned, and the detached dorso-median plates from the Genesee, may be 

 referred provisionally to D. minor. 



Comparative measurements of certain derm-plates for several species 

 of Dinichthys are exhibited by the table on the following page. 



Besides the plastron just described, there are several other interesting 

 structures preserved on the same slab. In advance of the plastron are 

 a number of badly weathered fragments, which evidently represent the 

 dorsal plates of the body. The forward portions of both antero-ventro- 

 laterals are covered over, and their proper boundaries obscured, by some 

 of these fragments ; but none of them are identifiable with certainty 

 unless it be the antero-lateral tip of the dorso-median, which rests upon 

 the angle of the right antero-ventro-lateral. This concealment of the 

 underlying plates along their margins is unfortunate, since the restored 

 anterior boundary has not such a clear basis of fact as one could wish 



