222 MR. W. K. PARKER ON THE OSTEOLOGY 
Let us look at this a little more closely. If ossification were to take place in the 
substance of the largely developed palato-quadrate cartilage of the Shark, it is very 
probable that no metapterygoid would be formed; for in that fish the posterior crus is 
extremely short. The shortness of the hinder limb of this cartilage is then repeated in 
those ossified vertebrate skulls in which the os quadratum terminates the series of 
bones posteriorly. Just below the articular head of the Tinamou’s os quadratum, and 
on its outside, there are some large air-passages (Pl. XL. fig. 3) ; these may also be seen 
in Ostriches and other Birds. There isa very large passage of this kind in the Lapwing 
(Pl. XXXVIL. fig. 1, q.). The air enters from the tympanic cavity, and the os qua- 
dratum thus becomes an extension of that cavity. In the smaller Turtles and Tortoises 
the os quadratum is the ear-drum ; the foramen just mentioned in the Lapwing is still 
larger in them, and has the membrana tympani attached to its margin ; the whole 
bone is hollowed out completely, whilst external and posterior to the upper articular 
surface it is developed, with the help of the hollow squamosal, into a very beautiful 
** bulla tympani.” 
I have purposely trespassed in the matter of the Tinamou’s os quadratum, but the 
interest of its bearings and meaning is not at all exhausted by what is written above. 
The mandibles of the Tinamou (Pl. XL. figs. 3,6,&7) are struthious throughout: a 
comparison of the figures given will show how remotely they are akin to those of the 
Pigeon, Plover, Grouse, Hemipod, or Sandgrouse (Pls. XXXIV.-XXXVII.). The 
membranous space in the thick part of the ramus is filled up, and the sutures have 
vanished from that part of each jaw which underlies the most coalesced portion of the 
palate and the zygoma; it is as though some special influence had passed through 
the middle of the Tinamou’s head, causing all the central part to become solid, whilst 
the fore and hind part of the head was left but little affected. This increased solidity 
and the narrowness and comparative delicacy of the jaws make us again to remember 
what exists in the Apteryx. In old Rheas the dentary part of the jaw (Pl. XLII. fig. 4, d.) 
is only retained to the articular (malleal) part, with its splints, by membrane, and the 
membranous space is persistent ; it vanishes in the Emu and Apterye. Each dentary 
(Pl. XL. fig. 3) elegantly repeats the lateral grooves and the vascular honeycombing of 
the premaxilla. This rich perforateness is seen both above and below; and a fragment 
of this part would determine the struthiousness of the bird, and prove it not to be 
gallinaceous, as much as a very small piece of foolscap paper would suffice to show 
that it had not been torn from a scroll of parchment. The extent of the symphysis is 
3 lines—relatively rather less than in the Rhea and Emu; it has become thoroughly 
anchylosed. The internal or anterior facet on the ‘‘os articulare” (Pl. XL. fig. 6) is 
roughly oval in outline, and is deeply scooped for the very convex condyle on the os 
quadratum. The posterior or outer facet runs up to the edge of the surangular inside, 
and is narrow kidney-shaped. Between the two facets there is a large pear-shaped 
hollow, lined with periosteum only; on the top of the angular and abrupt internal 
