DEVELOPMENT OF THE SKULL IN THE PIG. 295 
hammer; the solid rod itself develops for a while, but by the time of birth has shrunk 
into the feeble, pointed “ processus gracilis.” 
Fourth Arch, or Second Postoral.—At present this arch is extremely like the one in 
front of it (Plate XXVIII. fig. 5, and Plate XXIX. figs. 5 & 6, hy.); but it is flatter, 
and the right and left bars meet more closely and at an obtuse angle; its shoulder, 
also, is more upturned. This arch has been cut through in the palatal vertical views 
(Plate XXVIII. figs. 4 & 6, hy.); but the form of its tuberculum and capitulum are 
best seen in the horizontal section (Plate XXIX. figs. 7-10, hy.), and its shoulder or 
tuberculum is exposed in the sliced head (Plate XXIX. fig. 6, hy.). In this latter figure 
it is seen that the shoulder stands out like that of a Bird’s rib, the head or capitular 
portion thrusting itself as far inwards as it can on to the periotic wall. The /andmarks 
exposed in this figure are the portio dura and the top of the jugular vein (Plate XXIX. 
fig. 6, hy., 7%, j.v.). In figs. 7, 8, & 10 of the same, Plate, the horizontal sections show 
that the head of the hyoid growing towards the auditory mass is exactly like the head 
of the mandibular rod. ‘The portio dura nerve is seen both at its entrance into and its 
exit from the facial wall in this figure, and it is of the utmost consequence to the morpho- 
logist as being a most safe landmark. In the outer lateral view it is seen escaping 
behind that part of the hyoid rod which becomes the “ stylohyal” (Plate X XTX. fig. 6 
hy., 7). 
In one horizontal view (Plate X XIX. fig. 7, hy., 7°) its whole auditory course is seen, 
’ 
on one side its entrance into the wall in front of the first postoral cleft, and its exit 
behind the hyoid in the other; the same thing is shown in Plate XXVIII. fig. 8, 77 
(see also Plate X XIX. figs. 8-10, 7"). In most of these figures the head and neck of 
the hyoid are shown from above (Plate XXIX. figs. 7, 8) and from below (fig. 10, hy.) ; 
but in another seen from above (Plate XXIX. fig. 9) the section is through the rods 
a little lower down; and here we get a most instructive view, the shoulder evidently 
becoming dislocated from the neck, a process which will go on to complete separation 
of the parts. 
Fifth Arch, or Third Postoral.—In this arch the Mammal has developed merely the 
counterpart of the “ hypobranchial” segment of the first branchial arch; it is shown 
in a subhorizontal section 7m situ attached to-the larynx (Plate XXIX. fig. 5, th.h., lv.), 
and in the diagrammatic figure (Plate XXVIII. fig. 5, th.h.) is seen beneath the audi- 
tory sacs. In my paper on the Frog (Plate vit. p. 171) I showed how that the thyrohyals 
were the hypobranchial remnants of the first and second branchial arches developed 
backwards; those of the Mammal are therefore strictly homologous with those of the 
Frog, the latter being formed by retention of a part, which part is alone developed in 
the former. 
Looking again at the five pairs of facial arches as a whole, we see that the only arch, 
at present, which has developed a conjugational keystone piece is the first or trabecular: 
this is the “ prenasal rostrum” which figures so largely in my former paper on the 
Bird’s Skull. No other keystone appears afterwards in the Pig, save in the last pair; 
