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 p:\lato-quadrate cartilage of the Shark, it is very 

 probable that no metapterygoid would be formed ; for in that fish the posterior eras 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 (PI. XL. fig. 3) ; these may also be seen 

 in Ostriches and other Birds. There is a very large passage of this kind in the Lapwing 

 (PI. XXXVIL fig. 1, g.). 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 (PI. 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 (Pis. 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 (PI. 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 Apteryx. Each dentary 

 (PI. 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 " (PI. XL. fig. 6) is 

 roughly oval in outhne, 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 



