the post-orbital fontanelle, the foremost being the smaller bone ; these help the orbital plate of the frontal to cover the eyeball with bone. In front of the doubly notched ethmoidal wall the face is connected with the skull by a narrow band of cartilage, which is never quite severed in this type, Thus the fore face lifts on the skull in har mony with the depression of the mandible, by means of the elasticity of the parts, for, as we have seen, the firm splints above nasals and nasal processes of the pre- maxillaries are let into the frontal by their fibrous, lathy
ends.
Cranium of Fowl—Eighth Stage—Chicks Three Months old.—Many sutures still remain at this stage, but those between the occipital and periotic elements are fast filling up (Phil. Trans., 1809, plate 85, figs. 4-7, p. 794). Wemustrefer the reader to the work above cited for the details ; no stage shows the exquisite architecture of the ornithic skull more lucidly than this. The synchondroses are reduced to fine lines or sutures, and the size of the object is of great importance, as tending to make a difficult study easy to the observer.
Cranium of Fowi—Ninth Stage.—In chickens of the first winter, eight or nine months old, nearly full-sized, but yet succulent even in their skeleton, many things are to be observed (op. cif., plates 8G, 87, figs. 1-3, p. 795).
On the roof the sagittal suture is only obliterated in the parietal region, arid the super-occipital still retains an uppernotch. The occipital and auditory regions have entirely coalesced the opisthotic Avith the exoccipital first, and then with the prootic ; but the little epiotic seems to melt into the common mass of the ankylosing super-occipital and prootic, without any precedence either way. All the sutures across the cranio-facial hinge are still visible, namely, those made by the frontals, lachrymals, nasals, and nasal processes of the premaxillaries, in their relations. Where the frontals diverge by narrowness in front, behind the nasal processes of the premaxillaries, there the ethmoid is seen becoming fast bony from the substance of the perpendicular p!ate> and not by a separate upper bone, as in the Struthionidcc.[1] As to the facial bones, they yet retain much distinctness, and the prenasal and Meckelian rods still linger. The articular end of the latter rod is now ossifying fast, the two angular processes, so large in this type, are now bony. The fast coalescing roof and the coalesced floor are now of great thickness, and the diploe in this type is coarse (op. cit., plate 86, fig. 14). In all these growing stages, tracing bone by bone, as it appears, we have not yet met with the presphenoid, nor seen the be haviour of the great ethmoidal wall in relation to the hinder skull. In the most advanced winter chickens these things are to be seen (fig. IS, p.s., p.e., b.s.)
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Fio. IS. Skull of young Fowl of first winter, ninth stage, side view, x lj
diameters, o.s. 1 and o.s. I, the two small orbito-sphenoidal centres above the small preaphenoid p.s., which is only partially ossified at present. The sutures at this stage are very instructive. The fore-face has been removed, and the anterior edge of the perpendicular ethmoid (p.e.) is the posterior boundary of the cranio-facial cleft.
The presphenoidal region is merely that band of car tilage which lies partly above, but mainly behind, the fenestra (i.o.f.). A small ossicle has appeared in it close below the second orbito-sphenoid (p.s., o.s. 2), the foremost orbito-sphenoid (o.s. 1) has coalesced with the perpendicu lar ethmoid.
From that bone the rostrum of the parasplienoid is still distinct (r.b.s.), but the perpendicular plate has now reached the basi-sphenoid (p.e., b.s.), and between them, and below the still soft lower part of the presphenoid, there is a high vertical suture. This suture, and this steep bony wall beneath the presphenoid, are of the greatest interest to the morphologist. We saw that the basi- sphenoid was compound, having in it, besides the para- sphenoidal elements as investing parts, both the para- chordal ends and the trabecular apices. So it is, for the Bird s skull runs over, or rather is built upon, the mar vellously metamorphosed first visceral arch the arch formed by the primordial " trabeculas cranii." Counting from the spine, we have three cranial sclerotomes in the osseous stage. The first is formed on a notochordal and parachordal foundation ; this is the " basi-occipital." The next is formed on a foundation partly parachordal and partly facial the basi-sphenoid. The third is the " pre sphenoid," and it is tilted up over the forth-growing trabecular arch, the elements of which early coalesce at the mid-line, and the common crest of which is not, for a long time, in any way actually separate from the approximat ing roofs of the nasal sacs.
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FIG. 19. Skull of an old Fowl, tenth stage, X li diameters, upper view. A1:rvo
the line from n. is seen the large two-spurred nasal, the processes of which upper and lower are marked n.p , the cartilaginous structures of the noso are not figured.
intense degree of ankylosis, and yet certain sutures are persistent to old age, or at least show some chink or mark of their original separateness. In this the Fowl agrees
with most birds ; but, being at no great height above the- ↑ Phil, Trans., 1866, plate 8, fig. 10, eth., p.9.
