374 RAMSAY H. TRAQUAIR ON THE 
extending from above downwards and slightly forwards, with a flattened and 
somewhat expanded upper extremity, below which it is suddenly constricted, 
whereupon it once more, though very gradually, increases in diameter towards 
its lower extremity, which is, however, not well seen. Placed along its 
posterior margin is the operculum, a high narrow plate, with rounded posterior- 
superior angle and gently curved posterior margin. Below, it overlaps the 
interoperculum (¢.op.) a much smaller plate, whose anterior-inferior angle is 
somewhat produced so as to pass down close to the posterior extremity of the 
mandible. In front of these two bones, and covering the hyomandibular 
extremity, as well as a portion of the cheek, is the preoperculum (p.op.), a plate 
of a somewhat triangular form, whose three margins may be designated as 
posterior, anterior-superior, and anterior-inferior, and its angles as superior, 
inferior, and anterior. The posterior margin, the longest, is gently convex, and 
follows the contour of the anterior margins of the two preceding plates with 
which it is in close apposition ; the anterior-superior margin is in contact with 
the suborbital chain, the anterior-inferior one with the maxilla; the superior and 
inferior angles are acute, the anterior one very obtuse. On its internal surface 
(fig. 8) a fine ridge is seen connecting its superior and inferior angles, which 
corresponds with a slime canal traversing its interior on its way to the mandible, © 
In none of the numerous specimens which I have examined is the palato- 
quadrate apparatus exhibited, a fact which may be accounted for by the heads 
being almost always crushed quite flat, and the parts in question covered up 
by the large external facial plates. ; 
The maxilla (mz.) is of a broad triangular shape. Its inferior margin is gently 
convex, so is likewise the posterior one, which is in contact with the preoper- 
culum ; the superior margin slopes downwards and forwards to the anterior 
extremity. Its external surface is sculptured with fine vertical strive, save on — 
a small area distinctly marked off along the superior margin, deepening towards 
the extremity of the bone and overlapped by the large anterior suborbital. The 
mandible is weak, slender, tapering, and gently curved with upwardly directed 
concavity ; its constituent elements, with the exception of the dentary, are not 
recognisable. From below the interoperculum a set of narrow branchiostegal 
_plates extends on each side between the ravine of the mandible; their exact 
number cannot be determined, though I have counted at least six. 
The orbit (07) is placed high up and far back on the head, its position being 
right above the articulation of the mandible. There is evidence that it is sur- 
rounded in the first place by a complete ring of very narrow osseous plates, 
besides which there are, as in the other members of the family, two outer sub- 
orbital plates (s.o.). One of the latter set, narrow and somewhat curved, lies 
along the posterior-inferior aspect of the orbit, being also in contact with the 
anterior-superior margin of the preoperculum, the other of an oblong shape 
