206 TJie American Geologist. October, i892 
tress or support externally for the great "shear-tooth" so charac- 
teristic of the genus. On its inner face this arm also carries an 
upwardly concave, horizontal plate which formed the floor of the 
orbit, enclosing, with the pre- and post-orbitals, the cavity for 
the eye. In front the sub-orbital formed a sutural connec- 
tion with 
(12.) The Fi-emaxilhtri/. This plate, which has been hith- 
erto unknown, is not found in the specimen here described, but 
its presence has been established from others. A complete de- 
scription of it will be given in a later note, as its relation to the 
teeth would require the introduction of the subject of the denti- 
tion. For the present it must suffice to say that it completed 
the outline of the fore-part of the head, enclosing the nasal open- 
ings and filling up the recess observable between the nasal and 
the pre-orbital plates. 
The great shear-tooth set in the upper jaw on the lower face of 
the sub-orbital is shown in the diagram, but it has been so fully 
and clearly described and figured by Dr. Newberry that nothing 
further need be added here concerning it. 
(13.) The Post-ma n/iitaf plate, figured by Dr. Newberry in his 
diagram, is not found in the present specimen, nor are its rela- 
tionships 3'et clearly known. It apparently formed one of several 
thin blades of bone whose office was the connection of the lower 
jaw with the head and which are usually found in so broken and 
confused a condition that their interpretation and re-construction 
have not hitherto been possible. 
The principal "slime-canals" have been indicated. These are 
four in number. Three of them arise nearl}' together, about the 
meeting point of the supra-occipital, parietal and frontal plates. 
Of these, one which may l)e called the pre-orbital canal, runs 
almost straight to the anterior edge of the skull. It has not ^^et 
been traced on to the premaxillary plate. Another, which de- 
serves the name of pre-orbital, runs to the central region of the 
plate of the same name where, slightly curving, it passes to the 
margin and is continued on to the sub-orbital plate to meet the 
sub-orbital canal. This runs from the hinder and lower angle of 
that plate forward to its junction with the premaxillary plate and 
will doubtless be found continuous there with another when that 
plate is better known. The occipital canal starts nearly with the 
first and second and runs outward and l)ackward to the lock-joint 
