176 RAMSAY H. TRAQUAIR ON 
about 4 inch in length, so that I rather think we have here a fragment of the 
anterior extremity of the dentary bone of the other side of the head, with the 
sympbyseal laniary. 
Returning to the examination of the smaller specimens, a portion of the 
splenial element is seen in one specimen, exposed by the breaking out of a 
portion of the middle of the mandible. The articular element, which was 
doubtless also present, is not exhibited in any specimen I have seen. 
The opercular bones are largely developed. The operculwm (op. fig. 2) is a 
large, somewhat square-shaped plate, though broader above than below, and 
behind than in front. Its posterior-superior angle is rounded off; its inferior 
margin overlaps another plate, which may be considered to be the suboperculum 
(s. op.). This is somewhat narrower, and has its posterior-inferior angle much 
rounded off; its upper and lower margins are nearly parallel, and from the 
former, just at the anterior-superior angle of the bone, there projects a short 
pointed process, producing the anterior margin a little way upwards. 
In front of the operculum, and covering a large part of the cheek, is a plate 
(x) of a somewhat oval shape, and somewhat obliquely placed, so that its long 
axis runs from below upwards and forwards. Above, it is in contact with the 
outer edge of the cranial shield; its posterior margin is separated from the 
operculum by a smaller plate (p.op.). The latter is of a narrower shape, rather 
pointed above and a little less so below ; its long axis is pretty parallel to the 
direction of the hyomandibular which it covers ; its posterior margin, in contact 
with the operculum, is gently convex ; its anterior one, somewhat angulated, 
articulates with the large plate 2, and below also with the smailer one wa’. 
This third plate z lies immediately above the articular extremity of the 
mandible ; its posterior margin, covering the lower extremity of the hyoman- 
dibular, is in contact with the suboperculum below, touching also the plate 
p.op. above ; its upper margin is articulated with the plate 2, while in front it 
comes into relation with the oblique posterior margin of the maxilla. As 
figured by Acassiz, three precisely similar plates occur in the same position in 
Megalichthys,* of which he compares both the upper and posterior to the 
so-called pree-operculum of Polypterus, while the lower one he compares to the 
little bone fixed above the posterior edge of the maxilla in the Salmonide, &c., 
and which by Mr Parker is considered to be the homologue of the malar bone of 
other vertebrata.t In Osteolepis, according to PANDER, the corresponding space 
on the cheek is occupied by one large plate, denominated by him “ preeo- 
perculum,” on which, however, lines are visible indicating a division into three 
similar component parts. On comparing the arrangement with what is seen in 
* “ Poisson’s Fossiles,” vol. ii, part 2, p. 92; ‘ Atlas,” vol. ii. pl. Ixiii.a, figs. 1 and 3, 7, &, J. 
+ “On the Structure and Development of the Skull inthe Salmon” (Salmo salar, L.), Phil. Trans., 
1872, p. 100. 
