346 PROP. W. K. PAEKEE ON ^GITHOGNATHOFS BIEDS. 



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 aerial vibrations. 



Above the " Pisces Dipnoi," as soon as vee 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-di'um, 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, pis. 29-32), by Professor Huxley : 

 this type is Menohranchus 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 vrith 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, pis. 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 ^ara/iew/'a^ elements). This aborted fore end of the axis, the ear- 

 sacs, and the visceral or pleural arches are " by joints and bands 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 %vith 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, p'), there 

 is no discontinuity of tissue between the pedicle of the mandibular suspensorium and 

 the trabecular band ; but soon afterwards ("Fowl's SkuU," pi 81. figs. 1 &2) these tracts 



