30 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
ventro-median, 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 
corresponding 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. intermedius, aud 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 1.14, and, assuming 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. newberryi a mandible meas- 
uring 284 cm. in length.” In the same horizon are also found de- 
tached dorsal shields which are considered by this writer as belonging 
to D. newberryi, 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. newberryi 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. newberryi, and the 
dorsal shields from the same horizon as the last, all belonged to a single 
species. The correspondence of parts is such as to permit of a theo- 
retical association of the plastron with the mandibles of D. newberryi, 
but 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. VII. p. 626. 
2 Clarke, J. M., Bull. U. $. Geol. Survey, No. 16, p. 17, 1885. 
