PROFESSOR OWEN ON THE GENUS DINORNIS. 137 
mandible. The surangular (29), coalescing or connate posteriorly, like the angular (30), 
with the articular element, bounds the outer ramal depression above and terminates 
forward by penetrating the upper division (32’) of the dentary, with which it thus unites 
as by a species of gomphosis. ‘The splenial (fig. 7, 31), conversely with its condition in 
Alca impennis and many other birds, has coalesced posteriorly with the angular and 
surangular, and terminates in a free point anteriorly, being lodged, behind, between the 
angular and surangular, where it closes internally their interspace, and anteriorly 
between the two divisions of the dentary. Of these the lower (figs. 1 & 7, 32”), as in 
D. robustus and D. crassus, is longer than the upper one. 
The outer surface of the unbranched part of the dentary and the symphysial part 
determining the contour of the lower mandible repeat pretty closely the characters 
described and figured in D. robustus. The alveolar groove (fig. 5,0) is narrow and 
multiperforate. 
The length of the cranial cavity is 2 inches, its extreme breadth (across the hind 
third of the prosencephalon) is 1 inch 8 lines, its extreme height (behind the sella near 
the fore part of the epencephalon) is 1 inch 2 lines; and these admeasurements give 
almost accurately the dimensions of the brain in a bird which, weight for weight, 
equalled or surpassed the Rhea americana. The cavity is nearly equally divided length- 
wise between the ep- and pros-encephala. A low tentorial ridge forms the boundary, 
arching from above, vertically, along the side walls of the cavity, interrupted at the fore 
part of the petrosal sinus, and subsiding on the floor of the cavity, after bounding 
anteriorly the triangular depression for the optic lobe. From the lower and back part 
of this depression is continued the canal for the main part of the trigeminal nerve 
(foramen ovale) traversing and determining the alisphenoid neurapophysis. ‘The upper 
semicircular canal bounds below and gives the arched curve to the petrosal sinus. The 
roof of the epencephalic chamber is less arched from before backward than in most 
birds, owing to the hinder position and almost verticality of the foramen magnum and 
to the degree in: which the superoccipital inclines forward to join the parietals. The 
tentorial ridge shows no trace of the tumid swelling which characterizes it in the Didus 
ineptus'. At the lower lateral part of the epencephalic wall is the multiperforate 
shallow depression receiving the acoustic nerve. Behind this is the canal for the vagal 
nerve and entojugular vein: below this opens the small precondyloid foramen. ‘The 
prosencephalic chamber has its side and upper walls divided by a long, low, smooth, 
broad rising, which arches, from the tentorial ridge above the fore part of the petrosal 
sinus, obliquely forward and a little downward, subdividing at the rhinencephalic 
chamber: it indicates a corresponding longitudinal furrow of the cerebral hemisphere. 
In all Dinornithes, and in the ratio of their size, the walls of the cranial cavity are 
thick, mainly through the abundance of largely cellular diploé interposed between the 
thin and compact outer “table” and the thicker compact inner table of such walls. 
‘ Memoir on the Dodo, Zool. Trans, vol. vi. p. 71, pl. 23. fig. 1, 0. 
