THE CRANIAL OSTEOLOGY OF RHIZODOPSIS. 171 
The anterior, or fronto-ethmoidal division of the cranial shield is not so 
well preserved, so that it is not possible to map out its constituent ossifications 
with completeness ; in no case are its external or orbital margins well defined, 
and its upper surface is more or less broken and crushed. Nevertheless, the 
form and constitution of its anterior margin are unmistakeable. This is 
crescentically expanded, forming the rounded depressed snout ; and to the two 
dentigerous bones, the premazille forming its oral edge, we shall presently 
return in describing the bones of the jaws. I have not been able to detect the 
nasal openings. 
The external surfaces of these cranial plates are ornamented with minute 
tubercles and short ridges, frequently arranged in lines radiating from the 
centres of ossification. 
Facial Bones.—Immediately behind the posterior margin of the cranial 
shield are the usual three plates (s.¢., fig. 1), one median and two lateral, which 
are of such constant occurrence in fishes of the Rhombo- and Cyclodipterine 
families. I have already, in my memoir on the structure of Tristichopterus 
alatus,* expressed my opinion that these are equivalent to the transverse chain 
of supra-temporal ossicles in Polypterus, Lepidosteus, &c. 
The hyomandibular is a somewhat elongated bone, extending downwards 
with a slightly backward inclination from below the squamosal to just behind 
the articulation of the lower jaw; it is also slightly curved, the concavity being 
directed forwards. Above, where it articulates with the cranium, it is flattened 
for about a little less than one-third of its length; this flattened portion, to 
which the superior anterior angle of the operculum is articulated, becomes very 
suddenly cut away on the posterior aspect, below which the bone becomes 
slender and cylindrical, expanding, however, in thickness in its lower half. 
Remains of a powerfully developed palato-quadrate apparatus are seen in several 
specimens, but not exposed with sufficient completeness to admit of any de- 
scription of its component elements ; its outer margin is for some distance 
articulated with the inner aspect of the maxilla, behind which it recedes a little 
inwards to admit of the passage of the masticatory muscles to the coronoid 
part of the lower jaw. | 
By reason of the slightly backward slope of the hyomandibular, the gape is 
wide, and in three specimens, it is exposed all round the head, so that the 
bones forming the edges of the mouth are very completely seen. In nearly all 
the heads preserved in nodules the upper margin of the mawilla (mz. fig. 2) is 
injured, but its complete contour is well exhibited in detached shale specimens. 
In shape it resembles very closely the maxilla of Megalichthys, being of an 
elongated triangular form, broadest about the junction of its posterior and 
middle thirds, and narrowly tapering anteriorly. Its posterior extremity forms 
* Trans. Roy. Soc. Edinburgh, vol. xxvii. (1874) p. 386. 
