
O F TRISTICHOPTERUS ALATUS. 387 
opercular bones, and covering a great portion of the cheek, is a large oblong 
plate (p. op), which comes into close contact below with the hinder part of the 
maxilla, and articulates in front with two smaller plates separating it from the 
orbit, while above it is in contact with the side of the posterior part of the cranial 
shield. Its outer aspect is granulated; the inner is smooth, and shows, along 
its oblique and slightly curved posterior margin, a narrow shallow groove, evi- 
dently for articulation with the hyomandibular element of the suspensorium. 
The correspondence of this plate with the similarly placed one in Osteolepis, 
Diplopterus, Gyroptychius, &c., and with the great cheek plate of Polypterus, 
is at once evident. In those fishes it is by most authors reckoned to be the pre- 
operculum, though by Professor Huxiey it has been marked “ supra-temporal,” 
seeing that by its remarkable extension forwards on the cheek it differs so 
greatly from the true preoperculum in the Lepidosteid Ganoids, and in ordinary 
Teleostei. The orbit is situated very far forwards, and has connected with it 
a distinct chain of osseous plates (s. 0, figs. 1 and 11). Of these there is a small 
one placed at the posterior superior angle of the orbit, lying for a small distance 
along the outer margin of the anterior division of the cranial shield, immediately 
in front of the transverse line, which marks that off from the posterior division. 
This is followed by two plates of considerable size, forming the posterior bound- 
ary of the orbit, and interposed between it and the great cheek plate. Of these 
the lower one is the larger; it is in contact below with the maxilla, and sends a 
process forwards forming a portion of the inferior boundary of the orbit, when 
it apparently joins another, or pre-orbital, plate, whose conformation is not, 
however, very distinctly seen. The mazilla (mx) forms a long and narrow bar, 
slightly expanded posteriorly, placed below the great cheek plate and the sub- 
orbitals, and extending forwards to the small premawaila, which is firmly united 
with the anterior, or ethmoidal, part of the cranial shield. The oral margin of the 
maxillary bone is distinct enough, though in no specimen is its upper boundary 
shown with the clearness that might be desired. Internally, along its anterior 
two-thirds, it was certainly very firmly united to the outer margin of the palato- 
quadrate arch, an interval being left posteriorly for the passage of the muscles 
of the lower jaw. 
The palato-suspensory apparatus (p. g, figs. 2, 3, and 10) is formed by a broad 
and extensive bony lamina, extending forwards from the opercular bones, and 
| articulation of the lower jaw to the ethmoidal part of the skull. Its upper margin 
| is connected posteriorly with the squamosal region of the cranium; in front of 
| this its plane becomes a little twisted, so that its outer surface comes to look 
more upwards, and the previously upper margin is directed inwards to the base 
of the skull. Its posterior margin, gently curved with a posteriorly directed con- 
vexity, passes obliquely downwards and backwards to the very posteriorly situated 
articulation of the lower jaw; this margin, apparently corresponding to the hyo- 
VOL. XXVII. PART III. 51 
