DEVELOPMENT OF THE SKULL IN THE OSTRICH TEIBE. 133 



hyals {th. 1, th. 2) (1st branchials) articulating on each side of its somewhat broadened 

 end. The third poststomal is, as usual, composed of two rods on each side ; ossification 

 has far advanced in the larger proximal rod. The upper thyro-hyals, the basi-, and the 

 cerato-hyals are not at present in the least ossified. 



Having despatched the primordial parts of the Ehea's skull and face, I turn to the 

 opercular or splint-bone series. Most of these have already assumed very much of their 

 persistent form, but they are entirely unanchylosed ; so they thus present us with an 

 extremely valuable series of objects for comparison with other vertebrates. 



The upper, marginal, and orbital regions of the frontals (Plate IX. figs. 3-6,/.) are 

 well formed, and the bone is acquiring a considerable thickness ; the same may be said 

 of the parietals (p.) ; only a trace of the two sagittal fontanelles can be seen (Plate IX. 

 fig. 5). The lacrymals and nasals (lateral and upper ethmoidal splints) are completely 

 formed {l.n.), the latter having no descending process (Plate IX. fig. 5, «.), thus difiering 

 from Struthio, and the former having a large fenestra in its antorbital plate (Plate IX. 

 fig. 3, I.). Ostrich-like, the nasal processes of the intermaxillaries are completely fused 

 (Plate IX. fig. 5, px.) ; they end in front of the broad upper ethmoidal bone, which con- 

 dition is not lost in the adult. Round that bone, the blunt styles formed by the posterior 

 part of the nasals creep for some distance on to the narrow, wedge-like ends of the fron- 

 tals (Plate IX. fig. 5). The large, extremely thin and splintery palatine processes of 

 the intermaxillaries (Plate IX. fig. 4, px.) are very remarkable, and they are separated 

 by a very clearly cut fissure from the marginal part ; into this fissure, behind, where it 

 widens, the pointed wedge-like end of the prevomer (Plate IX. fig. 4, pv.) fits, thus 

 obliterating the " anterior palatine foramen." 



The prevomers have their largest relative development in the Rhea, and their smallest 

 in the " Gallinae," " Tetraoninae," " Pteroclinse," and " Hemippdiinae " (compare the 

 figures in plates 34-36 with^v. in plate 42 of Zoological Transactions, vol. v.). 



The very perfect development of the ascending process (Plate IX. figs. 3 & 5, and 

 Plate X. fig. 14) makes this bone in the Rhea very valuable for comparison with 

 that of the Ophidian; and although the nasal gland is not enclosed between the 

 vomer and prevomer as in the snake, yet its duct so passes down outside the great infe- 

 rior turbinal as to pour the secretion out at the identical place in which it is discharged 

 in that reptile (Plate IX. fig. 3, d.n.g.). Moreover, the root of the ascending process 

 of the Rhea's prevomer is to be seen ascending over the edge of the deeply bifurcate 

 vomer (Plate X. fig. 14, v.), which is almost a double bone. This submesial portion of 

 the prevomer is in reality the proximal portion, which has its immediate relation to 

 the intermaxillary axis, as the middle splint ; all the rest of the bone, which has spread 

 out to such an exorbitant size, receives its explanation in the fact that in the Bird this 

 splint takes up the place which should for the most part be filled by the maxillary. 

 Serially, the prevomer is the homologue of the " inter palatine." A large irregular 

 fenestra is formed in this bone in the adult bird ; it is curiously, scooped below (Zoolo- 

 gical Transactions, vol. v. plate 42, pv.), and a notch has appeared separating the pala- 



