EASTMAN : THE DINICHTHYIDS. 29 



among all plates of the ventral ai'moring was one of simple overlap, as 

 iu Coccosteus and other forms. 



Ventro- Lateral Plates. — The inner margins of the autero-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 iu 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 ; ^ but however this may be, we are obliged to recognize the 

 existence of these two patterns or varieties of the plastron. The pres- 

 ent specimen, thei'efore, 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 their union was one of simple overlap ; 

 hence Dean's figures of D. gouldi (f) 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 surface of the 

 1 Amer. Geo!., Vol. SVIII. p. 317. 



