792 
ME. W. K. PAEKEE ON THE STEUCTUEE AND 
sphenoid; they do cause its ossification. The anterior bone (o.s., 1) is irregularly 
oval; the larger posterior piece (o.s., 2) is somewhat square in shape, and is fast 
wedged-in between the postero-superior margin of the presphenoid and the supero- 
anterior margin of the alisphenoid : these ossicles are still quite separable from the pre- 
sphenoid, although afterwards they set up ossification in it'*. 
If the narrowest part of the tract of cartilage above the interorbital fenestra were 
quite asborbed, it would then be plainly seen what should be considered the true mor- 
phological boundaries of the perpendicular ethmoid (p.e., eth.). This does take place 
in some birds, as in the Cormorant, the Sun-bittern, and the Stilt-plover ; and then the 
cleft also runs across to the common optic foramen, so that the small presphenoid is 
completely severed from the ethmo-basisphenoidal band below. In the Bird the pre- 
sphenoid is to be regarded as a wedge with its sharp end below, cleaving the basal 
region where the basisphenoid and vertical ethmoid meet : this is a remarkable modifi- 
cation of the Vertebrate skull. The frontal and perpendicular regions of the ethmoid 
are not separately ossified as in the Struthionidae (First Paper, p. 127 et seq.), but the 
ectosteal plates (right and left) work upwards through the frontal plate. At this stage 
the ethmoid is an irregularly oblong plate, finished in front but deficient above, below, 
and behind. The notched front margin is the posterior boundary of the great cranio- 
facial cleft. In the figure the isthmus of cartilage is seen as cut through ; below, there 
is a small hook of cartilage, the remnant of the lower boundary of the “ fenestra ” of 
the fourth stage ; this piece partly lies in the grooved fore end of the rostrum. The 
position, relation, and advanced development of the three great splints (f \,p., sq .) that 
wall-in the skull are shown in Plate LXXXV. fig. 1 ; anteriorly the frontal is seen to 
retire at the mid line, thus exposing the ethmoid above. 
An outer side view’ of this stage of the skull is shown in Plate LXXXV. fig. 2, vdiere 
the superoccipital is seen to be still quite separate, and the epiotic (ep.) nearly coalesced 
with the exoccipital. In the large sulcus behind the oblique cartilaginous selvedge of 
the prootic (pro.) is seen a wedge-like plate of bone (_ pt.o .), part of which appears on 
the outstanding edge of the exoccipital, in front of and below the remnant of the epiotic 
suture. This is the pterotic, the first ectosteal plate of which appears in the front and 
outer part of the “ lateral cerebellar fossa ” (fig. 1 , pt.o.) : its subsequent development is 
by endostosis ; at this' stage it has worked to the outer surface ; and here it appears 
behind the prootic (pro.), in front of and external to the epiotic (ep.), and above the 
opisthotic (op.). 
No one familiar with the skull of the Cod, Perch, or Halibut will fail to see how 
* On this ground a parostosis and an ectostosis meet, and nothing but the history of the development of the 
parts could have given any clue to the matter ; similarly, in the Mammalia, the “ cornua sphenoidalia ” or 
mesopterygoids crop up again as mere late ossifications of membranous tracts of the pterygo-palatine arcade ; and, 
instructively as to the case of the Bird’s double orbito-sphenoids, in Man the mesopterygoids are double on 
each side. 
