DEVELOPMENT OF THE SKULL IN THE OSTEICH TEIBE. 137 



tilage only. The orbito-sphenoidal laminae gre narrow (Plate XL fig. 1, o.s.), interme- 

 diate in size between those of the Rhea and the Ostrich ; but the presphenoid is not 

 so ornithic as in the former, being deeper and broader ; moreover it has, in accordance 

 with that almost constant correlation of size and precocity, already appeared in the form 

 of two oblong bony points on the right side, in the somewhat obtuse angle formed by 

 the giving off of the orbito-sphenoid. This appearance of the presphenoid as two linearly 

 arranged ossicles is a lacertian character, and breaks out here and there in the bird-class, 

 e. g. Pavo, Nisus ; but in the " Lacertilia " the anterior sphenoid is relatively very large 

 — large, as it were, at the expense both of the alisphenoids and vertical ethmoid, and its 

 development is by many centres of ossification. The interorbital " fenestra " is not half 

 so large as in the Rhea (if it exists at all in Stmthio it is but small and transitory) ; it is 

 ovoidal in shape, and only bordered by bone at top. This bone is the upper and poste- 

 rior part of the vertical ethmoid (Plate XI. fig. 1, p.e.), which is already large, and by 

 the absorption (or non-development) of cartilage, deeply and broadly notched above ; 

 this notch (Plate XI. fig. 1, i.e.s.) is converted into a fenestra by the upper (connate) eth- 

 moidal piece of bone, which is already closely adherent to the lower centre. The large 

 hinder half of the lower bone is thickened by a nearly vertical ridge in its upper two- 

 thirds, where the pars plana {p.p.) applies itself; above this is the descending plate of 

 the upper bone, and this is very thin, for it lies between the olfactory crura. The nasal 

 process of the intermaxillaries reaches the front of the upper bone, and the lower bone 

 has only reached this (vertical) line ; all in front of this is still cartilaginous, one flat 

 vertical plate reaching to the body of the coalesced intermaxillaries. In the groove 

 below, where these bones have completely fused, there is the merest trace of the inter- 

 maxillary axis, or prenasal cartilage (Plate XL fig. 2, px.), which is thus seen to disap- 

 pear much earlier in the Emu than in its congeners. But the fate of the axis of each 

 palatine arch is very different, its ossification being so early, and its importance as a per- 

 sistent bony part being so great. The anterior bone, the palatine (Plate XL fig. 2, pa.), 

 is at an unusual distance from its fellow, because of the very out-turned forks of the 

 vomer («.) behind; this is the case quite as much as in the Ouaran Lizard (Psamnio- 

 saurus scincus), but the long styliform ends of the pterygoids (p.g.) creep on to the 

 vomerine forks, and thus approach the mid line more closely. Each palatine is a low 

 triangle, the apex of which is the proximal (orbital) part, the anterior basal angle being 

 prolonged into a small style, fitting inside the inner retral process of the prevomer ; the 

 posterior basal angle is blunt, and lies outside the pterygoid (Plate XL fig. 2). The 

 basal part of this triangle is thick; the rest, or inner part of the bone, is thin and deli- 

 cately reticulate ; it will be absorbed considerably in the centre. The pterygoids (p.g.) 

 are thick and terete behind, flat and splintery for four-fifths of their extent ; an oval 

 sinuous condyle at the extreme end of each fits (by a synovial joint) on to the ptery- 

 goid facet of the os quadratum (Plate XL fig. 2, q.) ; but the facet on the pterygoid is 

 continued round the inner edge, because the lower half of the condyle is jammed in 

 between the os quadratum and the anterior pterygoid process (Plate XL fig. 2). The 



MDCCCLXVI. X 



