30 bulletin: museum of comparative zoology. 



veutro-mediau, which is preserved as far as its posterior apex on the 

 slab. 



"We have now to determine what species of Dinichthys is represented 

 by the ventral armor just described. In the absence of the dentition, 

 we must either associate the remains theoretically with mandibles of 

 con-espondiug size that occur in the same horizon, or must regard the 

 plastron as belonging to a new species. Fortunately, the proportions 

 between the different body plates are well known in D. terrelli and 

 D. intermedins, and from them we can readily compute the length of 

 mandible and size of dorsal shield to which the present specimen would 

 correspond. Thus, the ratio between the length of mandible and length 

 of the antero-ventro-laterals in D. terrelli^ is Lll, and, assurming that 

 about the same proportion held true for the species now under dis- 

 cussion, we should attribute it with a mandible 24 or 25 cm. long. 

 Now, from the Genesee Shales near Bristol Center, New York, J. M. 

 Clarke has described under the title of D. newherryi a mandible meas- 

 uring 28^ cm. in length.^ In the same horizon are also found de- 

 tached dorsal shields which are considered by this writer as belonging 

 to D. newherryi, although their dimensions correspond almost precisely 

 with those of D. minor. In fact. Dr. Clarke's tables (pp. 22, 23) show 

 that, while the mandibles of D. newherryi are about one half as large 

 as in D. hertzeri and D. terrelli, the dorsal shields are less than one 

 fifth the size of those in either species. Such a marked discrepancy of 

 ratio appears incredible in the light of comparison with other species ; 

 and the measurements of the plastron now under discussion militate 

 with the assumption that they, the mandibles of D. newherryi, and the 

 dorsal shields from the same horizon as the last, all belonged to a single 

 species. The con-espondence of parts is such as to permit of a theo- 

 retical association of the plastron with the mandibles of D. newherryi, 

 hut not with the dorsal shields that are referred by Dr. Clarke to this 

 species ; these latter being more properly assignable to D. minor, or a 

 species of equal size with D. minor. 



It must be borne in mind, however, that these conclusions depend 

 entirely upon empirical formulas ; they are therefore more or less tenta- 

 tive and provisional. It may be presumed from the general nature of 

 things, and in the absence of any contrary evidence, that the propor- 

 tions existing between parts of the derm skeleton were fairly constant 

 within the limits of one and the same genus. But the correspondence 



1 Wright, A. A., Report Geol. Surv. Ohio, Vol. VIL p. 626. 



2 Clarke, J. M., Bull. U. S. Geol. Survey, No. 16, p. 17, 1885. 



