MR. W. K. PARKER ON THE OSTEOLOGY OF BALZENICEPS REX. 273 
an oval shape in birds). The substance of the occipital bone is thick and richly 
cellular in the Baleniceps. 
In the Chick, the occipital sclerotome is developed from five centres ; one basal, a pair 
of laterals, and a pair of upper or ‘ epiotic’ pieces. The basi-occipital of the Chick may be 
seen, on the eleventh day of incubation, as a short rod of bone in the upper stratum of the’ 
cartilage of the primordial basis cranii; it encloses the tapering anterior extremity of the 
evanescent chorda dorsalis, but has not yet penetrated the thick cartilage beneath it, and 
is only just entering the substance of the, as yet, cartilaginous hemispherical condyle. . 
The basal centre lies on the same plane as the centre of ossification, which already 
occupies the posterior third of the basi-sphenoidal rostrum. The lateral or ex-occipitals 
may be seen at the same period as thin scales of bone of a somewhat crescentic shape, 
bounding the sides of the foramen magnum (hence their crescentic margin), sending 
upwards a process to join the epiotics, and another forwards and outwards to form the 
‘par-occipital’ ala, and to ossify that part of the occipital sclerotome which in the 
Chelonia exists as a distinct ‘ mastoid.’ 
On the eleventh day of incubation most of the upper occipital region is still cartilagi- 
nous ; but on each side of the mesial line, a little above the foramen magnum, a pair of 
small oval osseous centres already exist ; they are about a line apart. 
On the fourteenth day they are still nearly the same distance from each other ; but 
they are creeping up to the upper margin of the cartilaginous cranial wall, downwards 
nearly to the foramen magnum, and laterally they have begun to wall-in the superior 
semicircular canals. 
On the sixteenth day these pieces (called ‘ epiotic bones’ by Prof. Huxley, Croon. 
Lect. p. 13) have reached each other, as well as the upper margin of the occipital car- 
tilage and the superior boundary of the foramen magnum. 
On the nineteenth day only the upper and lower fourths of the line of union of these ~ 
‘ epiotics’ are visible, and the external margin has reached the external as well as the 
superior semicircular canal. 
In young Pigeons, a day after hatching, these ossifications have not commenced ; on 
the eighth day these rapidly-growing birds have a large single ‘ supra-occipital’ deeply 
notched in the middle, inferiorly, in the place of the oval membranous deficiency, or 
fontanelle, in the primordial occipital wall. In adult Pigeons, and also in the Dodo and 
Didunculus, traces of this structure still remain (see Strickland and Melville),—all the 
Columbine hitherto examined having a mesial ‘ supra-occipital’ foramen. These birds 
must be examined on about the third day for the separate epiotic pieces’. 
The broad smooth supra-occipital region of birds does not require a distinct inter- 
parietal, or central supra-occipital element ; but in the Chelonia, and probably in most 
1 As early as the third day after incubation, we find (from dissections made since the above was written) that 
in these typical birds there is only one osseous centre in the supra-occipital cartilage : it is shaped like a horse- 
shoe and is very rapid in growth. 
