28 ULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
two? And if two, what is their mode of union? We have no hesita- 
tion in answering that two median plates are present, and that the an- 
terior overlaps the posterior, as in Coccosteus. ‘The evidence appears 
perfectly decisive, and is of twofold nature; it depends upon a prom- 
inent surface elevation over the very region where we should expect the 
boundary between two median ventrals to be, and upon the fact that 
two centres of ossification are discernible. 
The surface elevation referred to is palpably of the same nature as 
those prominences which are formed by the hinder extremities of the 
antero-ventro-laterals where they are superimposed upon the posterior 
pair of ventro-laterals. All of these elevations are more or less eroded 
in the specimen, but the one: under consideration is scarcely more so 
than the others. If it were a purely fortuitous bulge of the surface, we 
should expect similar ones to occur elsewhere, whereas the prevailing 
aspect of the plates is flat and smooth. But inasmuch as the only re- 
maining elevations are found at those places where we know for certain 
that boundaries occur, and as this occurs at the only place in the median 
line where we should expect to find a boundary, we are compelled to 
look upon this as a significant, not an accidental feature. Moreover, 
the shape of the elevation corresponds with the tapering extremity of 
the antero-ventro-median, superimposed upon the posterior element ; 
and the outline of the latter is seen to be perfectly normal as compared 
with homologous plates, when we cut it off at this point. In fact, it is 
noteworthy that the shape of the postero-ventro-median bears a marked 
similarity to the bone last described (supra, p. 26), and shown in 
Plate 5, Fig. 1. 
But still more pertinent evidence as to the existence of two median 
ventrals is furnished by the structure of the plates themselves, It is 
apparent at a glance that in the postero-ventro-median ossification pro- 
ceeded from a single centre, which was nearly coincident with the centre 
of the plate itself. On holding the slab so as to reflect light at a proper 
angle, the course of vascular (Haversian) canals can be seen very dis- 
tinctly, especially at the right anterior boundary ; and all of these radiate 
toward the centre of the plate. Vascular canals are likewise apparent 
on the antero-ventro-laterals, but are only faintly perceptible on the 
antero-ventro-median. If the latter plate were articulated or fused with 
the posterior element, as in D. terrelli, it would be difficult to account 
for the significant elevation already referred to; and considering the 
relative thinness of the plates, such a mode of union could hardly have 
proved advantageous. It is more natural to suppose that the connection 
