546 PROF. W. K. PARKER ON GITHOGNATHOUS BIRDS. 
of hearing, the first postoral cleft being converted into the cavity of the ear-drum, and 
the contiguous parts modified behind this air-space to wall it in, and to perfect it for 
the conveyance of the aérial vibrations. 
Above the “Pisces Dipnoi,” as soon as we reach the Proteus and Siren, a stapedial 
plug is formed in the osseo-cartilaginous ear-ball; above these, many of the tailless 
Batrachia utilize the first postoral cleft for an ear-drum, and the parts of the face 
around this opening become profoundly modified and metamorphosed to perfect this 
new sensorial apparatus. 
But the simplest Amphibia are the best for comparison, as to their facial morphology, 
with the nobler types; and none of these is more instructive than the one treated of in 
the ‘Proceedings’ of this Society for 1874 (pp. 186-204, pls. 29-32), by Professor Huxley : 
this type is Menobranchus lateralis, one of the lowest of even the ‘“ Perennibranchiate 
Amphibia.” 
If the visceral arches of this form be carefully compared, as we slowly travel up 
through the types, with the Axolotl, Salamander, Frog, Chelonian, Non-carinate and 
Carinate Bird, we shall have a very adequate idea, at last, of the meaning of the multi- 
tude of bones that are to be found in the building of a bird’s face,—during the growth 
of its face, rather; for Nature, ready with her cementing “ osteoblasts,” is incessantly 
obliterating the once distinct and shapely stones by which it was gradually built. 
In the adult skull just referred to (Proc. Zool. Soc. 1874, pls. 29-31, pp. 186-204), 
the “chondrocranium” is in a condition of arrested ossification very similar to that of 
a chick at the beginning of the third week of incubation (see my paper on the Fowl’s 
Skull, Phil. Trans. 1869, plate 83). It may sound like a contradiction; but this skull 
is nearly all face; for the axial structures are exceedingly feeble, and end between the 
huge ear-sacs (hinder parancural elements). This aborted fore end of the axis, the ear- 
sacs, and the visceral or plewral arches are “ by joints and dands knit together” into a 
sort of ground-plan for the higher types of Vertebrate skulls. 
In these higher types the osseous metamorphosis, combined with the clefts (oblique, 
transverse, and longitudinal) that take place in the cartilaginous bands, or in their 
granular counterparts, give the results which we see in reptile, bird, or mammal. 
The most important binders, with their subdivisions and their changes, are formed 
by the tops of the mandibular and hyoid arches ; of the former only I wish to speak here, 
In the bird, the mandible is articulated to the skull by a huge bone (the quadrate) 
which is the dorsal part of that arch, the mandible itself being the ventral part. 
But in this the bird conforms to all the Vertebrata with the exception of the Lam- 
prey and his companions below, and the Mammalia above. In the early condition of 
the skull, whilst unchondrified (see Huxley, Elem. Comp. Anat. p. 138, fig. 57, ¥’), there 
is no discontinuity of tissue between the pedicle of the mandibular suspensorium and 
the trabecular band; but soon afterwards (‘‘Fowl’s Skull,” pl 81. figs. 1 & 2) these tracts 
