EASTMAN: THE DINICHTHYIDS. 29 
among all plates of the ventral armoring was one of simple overlap, as 
in Coccosteus and other forms. 
Ventro- Lateral Plates. — The inner margins of the antero-ventro- 
laterals are traceable with certainty throughout the greater portion of 
their length, but with a lesser degree of probability for the remaining 
(anterior) portion, where they are not only much abraded, but in part 
covered over by extraneous fragments, as will be noted presently. The 
boundaries of these plates are more sinuous than in any other known 
species, and their proportions with respect to the posterior pair are also 
different. But, as already remarked, the ventrals exhibit a greater range 
of variation, even within specific limits, than all the other plates of the 
body. 
One class of variations that deserves notice here is the relative length 
of the two sets of ventro-laterals. Sometimes the anterior pair is the 
longer, and again, apparently within the limits of the same species, the 
posterior pair exceeds them in length. 
Possibly these differences may have been correlated with sex, a greater 
portion of the abdomen having been protected in the one case than in 
the other ;1 but however this may be, we are obliged to recognize the 
existence of these two patterns or varicties of the plastron. The pres- 
ent specimen, therefore, belongs to that type of plastron which has the 
anterior ventro-laterals longer than the posterior. 
The external margin of the postero-ventro-laterals appears to have 
been evenly rounded. Unfortunately, the central portion of the plates 
has been eroded away, so that the contour of the inner margins can 
only be postulated. It is probable, however, owing to the tenuity of 
these plates, that the condition of thcir union was one of simple overlap ; 
hence Dean’s figures of D. gouldi ( have been followed in restoring 
their inner boundaries. Of the anterior borders of these plates, no 
trace whatever remains. There may be some significance attached to 
the fact that the antero-ventro-laterals are symmetrically worn away, 
their present eroded margins forming a regular curve from the ventro- 
median outward. Whether this symmetrical wearing away was in any 
respect influenced by the anterior margins of the hinder pair of plates 
may perhaps be questioned ; but at all events we must conclude that the 
former anterior boundary of these plates was not far from, and was prob- 
ably parallel with, the interrupted edges of the antero-ventro-laterals. 
That the plates in question were separated for a considerable distance 
posteriorly, is witnessed by an impression of the visceral surfaee of the 
1 Amer, Geol., Vol. XVIII. p. 317. 
