1848. | SIR P. EGERTON ON PTERICHTHYS. 305 
the positions they occupied in the bony investment of the body are 
easily discerned. The central plate (fig. 1, @) described by Miller as 
helmet-shaped has rather more resemblance to a Lottia or Patella with 
Fig. 2. 
the anterior margin truncated, the posterior one abruptly rounded. 
As compared with the lozenge-shaped plate of the same individual 
(fig. 2, g) it is full double its size, and in relation to the body of the 
fish occupies (as described by Agassiz in Pterichthys testudinarius 
and Milleri) at least half the entire area of the side with which it is 
associated. ‘The anterior truncated margin formed the boundary of 
the carapace, thus intercepting the union of the anterior lateral plates 
(fig. 1, ec) on the median line. These plates are pentagonal and arti- 
culate with the central umbonated plate a, the posterior lateral plates 
d, and the anterior ventro-lateral plates e, e. The anterior edge was 
hollowed out to allow the lateral motions of the head, and was 
strengthened by a marginal rim or thickening of the bone on the 
inner surface (fig. 1, 1). The posterior lateral plates d are also some- 
‘what pentangular, but the lower angles are produced to fit the anterior 
plates; they articulate with these, with the anterior dorsal plate a, 
the posterior dorsal plate 6, and the anterior and posterior ventro- 
lateral plates e, e, f, f. They are hollowed out on the posterior margin 
to give freedom to the lateral inflections of the tail, and are finished 
with a thickened rim (fig. 1, 1), and further strengthened by a rib of 
bone (fig. 1, 3) running diagonally in the direction of the largest 
diameter of the plate. The posterior plate 6 is somewhat similar in 
form to a, but smaller. It articulated with the central plate a an- 
teriorly, and with the posterior lateral plates d, d. In describing 
the under surface of Pterichthys latus, Agassiz says :—“ There ap- 
pears to have been no central plate on this side, but that the anterior 
