138 
ME. W. K. PAEKEE ON THE STEUCTUEE AND 
are growing more over the third vesicle and cerebellum. The roof-cartilage is thicker 
(fig. 10) and has developed an inturned selvedge, which encroaches upon the membranous 
orbito-sphenoidal region, whilst the “ mesoethmoidal ” wedge has grown further back- 
wards along the presphenoidal line. A thin spicule of bone has been formed over the 
“ ectoethmoidal wing” (figs. 7 & 9, l.e.), and this does not behave like the “ supra- 
ethmeidal” plate (fig. 6, n .*), but grafts itself upon the cartilage and becomes the so- 
called “prefrontal” bone. In the prosencephalic region I have not figured the per- 
manently distinct frontals above ; but laterally there are now a pair of new bones to be 
illustrated (figs. 6, 7, 9, 10, o . s .). These laminae are like the valves of bivalved “Ento- 
mostraca,” and occupy already nearly the whole of the orbito-sphenoidal space. These 
bones curiously illustrate, and are illustrated by, their counterparts in two very diverse 
types of Vertebrata: they have precisely the same character, as bones, as those deve- 
loped over the “ orbito-sphenoidal ” cartilage of the Sturgeon ; there, however, they con- 
tinue as distinct “investing bones,” even to old age (see adult Sturgeon’s skull, with soft 
parts modelled, in Mus. Coll. Surg.). In the Fowl, at the time of hatching, the orbito- 
sphenoidal region (“Fowl’s Skull,” Plate lxxxiv. figs. 7 & 8 , p . s .) is entirely membranous ; 
it has no alee growing from the presphenoid (ibid. Plate lxxxiii. fig. 11, o . s ., p . s .); but 
in a few weeks two membrane-bones appear on each side, and these soon graft themselves 
upon the presphenoid (see also Plate lxxxvi. figs. 11 & 14, and Plate lxxxvii. figs. 1 & 2, 
o . s ., p . s .). The single bone in the young Salmon grafts itself also upon the presphe- 
noid exactly in the same way (Plate V. figs. 6, 7, 9, 10, o . s ., p . s .) ; and not only so, but 
the upper edge of each plate splits and embraces the descending roof-plate (fig. 10). 
That this is not done in the case of the Bird also, depends upon the fact that the 
“ culmen cranii ” growing backwards from the “ ethmoid ” is arrested midway (ibid. 
Plate lxxxiii. figs. 2, 4, 5, Plate lxxxiv. fig. 7, Plate lxxxv. fig. 1, and Plate lxxxvi. 
figs. 11 & 14, o . s ., eth.). There is no difficulty now in understanding the meaning of 
the great interorbital bone of the adult Salmon (Plate VII. figs. 3, 4, 10, o.s.), nor in 
seeing why it is arrested in its growth downwards ; in like manner the distinct pre- 
sphenoid of the Fowl (op. cit. Plate lxxxvii. fig. 1 , p . s .) reaches no further downwards 
than to the top of the trabecular keel, in which the mesoethmoidal and basisphenoidal 
ossifications meet. Now we know that the primordial “ notch ” between the upper and 
lower retral median cartilages that grow from the ethmoid in the Salmon answers to the 
secondary “fenestra” of the Bird, it becomes a question of intense interest as to what 
the lower bar is, as well as how it comports itself. Here, in the Salmon, there is no 
median ethmoidal ossification ; there, in the Bird, the basisphenoid borrows its ossifying 
centre at first from the parasphenoid, a bone permanently distinct in the Fish, but the 
median ethmoid becomes an immense bone (op. cit. Plate lxxxvii. fig. 1, eth.), all arising 
from a single centre f. 
But the prepitnitary part of the basisphenoid is formed, as to cartilage, in the same 
* The “mesonasal” is lettered n by mistake in these figures (2 and 6); it is marked eth. in the rest. 
t Not in the “ Eatitae,” however (see “ Ostrich's Skull,” Plates yiii.-xiy.) ; here the roof is separately ossified. 
