134 MR. W. K. PAEKER ON THE STRUCTTJEE AND 



tine from the proximal process ; the palatine and zygomatic processes of the prevomer 

 are not far from parallel, and are nearly of the same length (Plate IX. fig. 4). The 

 proximal (submesial) part of the prevomer of the Ehea is much wider than that of the 

 Ostrich, and is less distinctly marked off from the body of the bone. To sum up the 

 regions of this huge bone — a bone so strongly limited as to its occurrence in a separate 

 condition in the classes and orders of the Vertebrata, but constant and attaining its 

 highest autogenous condition in birds — we have a body, a premaxillary, an ascending, a 

 proximal, a palatine, and a zygomatic process. 



The vomer of the Ehea (Plate IX. fig. 4, v) is exceedingly instructive, being very 

 large, and having its two symmetrical halves united merely by an isthmus at its middle, 

 only one-fourth the length of the bone, so that it has but narrowly escaped being double 

 — a condition not really wanting in the bird-class, e. g. in Numida meleagris and Musi- 

 capa grisola. 



The vomer of the Rhea is a very close counterpart of that of the Chelonians, a group 

 peculiar amongst reptiles in having this bone azygous ; for the middle part or septum of 

 the " middle nares " is high, compressed, and keeled below, whilst the hinder part is on 

 a higher plane than the front portion. The front part is divided by a deep, clean, oblong 

 notch, and the rami, grooved below, lie on the enormous palatine plates of the inter- 

 maxillaries; in the Chelonian the front part is unsplit, and is affixed by a transverse 

 suture to the short palatine portion of the intermaxillaries. The higher placed, out- 

 spread, posterior portion of the vomer in both the Tortoise and the Rhea, articulates with 

 both the palatines and the pterygoids ; the inner part of the maxillary of the Chelonian, 

 in relation to the vomer, just takes the place of the prevomer of the bird. 



Again, in this genus there is no maxillary splint-bone ; this part of the face is subject 

 to extreme reverses of development even when present, but its absence in perhaps more 

 than ninety-Jive per cent, of the ornithic genera, appears to have been hitherto unex- 

 pected and unlooked for. 



Both to Professor Huxley and myself the prevomer of the bird seemed to be a very 

 awkwardly fitting representative of the maxilla of the other classes, but it was not until 

 a rudiment of the true maxilla turned up to me in an unripe Emu-chick that the real 

 state of the case was understood. 



The anterior and posterior external splints of the pterygo-palatine arcade, viz. the 

 jugal and quadrato-jugal (Plate IX. figs. 2>-b,j.qj.) are very feeble, as usual, in the 

 Rhea ; the latter is extremely short and passes within the jugal, only appearing out- 

 side, close to its articulation with the quadrate bone. These outwardly-drawn splints 

 are persistently distinct (see Zool. Trans, vol. v. plate 42, figs. 1, 2, & 4i,J.qJ.). 



The proximal or suspensorial splint of the mandibular arch is not long a free bone, 

 but soon coalesces with the sides of the skull proper .and auditory capsule ; it is the 

 squamosal (Plate IX. tigs. 3-6, sq.), and is very large and batrachoid in this bird, as in 

 the other Ostriches ; and, as in the rest of this group, although not in other birds, it is 

 excluded from the cranial cavity, of which it only forms at any time a vicarious part. 



