THE CRANIAL OSTEOLOGY OF RHIZODOPSIS. 173 
lower margins are tolerably parallel save just at the anterior extremity, where 
the upper one bulges a little upwards in a slight convexity, and at the posterior 
extremity where the same margin suddenly slopes downwards and backwards 
at an obtuse angle, meeting the lower one, which likewise curves upwards 
towards it, in a posteriorly directed point. Nothing has been said in the works 
of previous writers concerning the constitution of the mandible, though it 
might be inferred to be a composite structure, as it is in all fishes with ossified 
skeleton, and more especially in the Ganoidei. In one specimen we find that 
over a considerable area the bony matter of the outer aspect has flaked off, 
leaving behind it a pretty sharp cast with sutural lines. On close examination 
a suture is seen commencing near the posterior extremity of the upper margin 
of the jaw, which, passing gradually downwards and forwards, marks off as 
dentary (d. fig. 2) an element precisely the counterpart in shape of the bone 
reckoned by Messrs Hancock and ArrTuey “ preemaxilla,” but here placed with 
its toothed margin wpwards instead of downwards as supposed by them. These 
two bones, right and left, are in many specimens indisputably seen forming the 
lower margin of the mouth and meeting each other at the symphysis. Each 
dentary bone is of a somewhat narrow and elongated form, truncated and some- 
what expanded at the anterior or symphyseal extremity, and pointed at the other 
or posterior. The upper margin, nearly straight, save just in front where it shows 
a slight convexity, is set with a single row of small pointed teeth of nearly uniform 
size, but the anterior extremity bears in addition a single more or less incurved 
laniary tooth, much larger than the others, and also more internal in its 
position; the opposite margin, thin and sharp, displays a gently flexuous 
contour. Seen from the inner aspect, the anterior extremity of the bone 
presents a conspicuous thickening, in which the large laniary tooth is socketed, 
and which at the dental margin passes into a delicate ledge, which runs back 
for some distance along the roots of the smaller teeth. The teeth borne by 
this bone are round in transverse section, slender-conical in shape, brilliantly 
polished, and apparently smooth externally, but under a lens the surface is seen 
to be delicately fretted with minute longitudinal groovings, disappearing 
towards the point ; the large laniary is also very distinctly fluted or plicate at 
its base. 
The rest of the outer surface of the mandible is composed of at least three 
additional bony plates, separated from each other by sutures which pass 
obliquely forwards and upwards. The posterior and largest of these (ag. fig. 2) 
covering over the articular region, may be considered as equivalent to the 
angular element, though it also occupies very much the place of a supra- 
angular ; the other two (d.) in front of the latter and below the dentary, 
may be called infradentary. The presence and contour of these large infra- 
dentary plates is perfectly clear, the evidence as to additional ones is obscure. 
