170 MR. W. K. PARKER ON STEATORNIS CARIPENSIS. [ Apr. 2, 
large in the Raptorial Dicholophus, and also in the Procellariide. 
In the Laridz it is smaller, and in the Alcide (Alca torda, Uria 
troile) it is a mere rudiment composed of one or two independent 
bony nuclei at the infero-external angle of the pars plana’. 
It is worthy of remark that the palato-quadrate arcade of the 
Ichthyopsida, although appearing here and there at hap-hazard, as 
it were, in the families, shows one part in the birds just mentioned, 
and another in the Passerines. In these latter birds I have found no 
distinct “‘ os uncinatum,” merely a knob or outgrowth of the 
pars plana representing that bone. But in all these culminating 
types there is a special apparent outgrowth of the palatine bone 
at its postero-external angle (see Trans. Zool. Soc. vol. ix. pl. lv. 
figs. 1,5, 6, and 13, ¢.pa.); this is formed by the independent 
ossification of a considerable part of true hyaline cartilage, which is 
in reality the reappearance of the horizontal part of the “ palato-quad- 
rate” bar of the Ichthyopsida. In Steatornis the form of the “ os 
uncinatum ” (0.u.) is that of an inverted T: the stem is attached to 
the antero-inferior edge of the pars plana, the front ray runs upon 
and is attached to the angle of the maxillary, and the hind crus 
is attached to the inner edge of its jugal process. In contemplating 
these things we are let down, so to speak, not merely to the 
Reptilian, but to the larval Amphibian level. The supraorbital 
chain of bones, seen in the Tinamous and some other binds, the 
sutures in the skulls of those Gallo-struthious birds, and the opis- 
thoccelian dorsal vertebree of many birds, only let us down to the 
Reptilian level. 
But the “os uncinatum,” the post-palatine, and the remarkable 
squamosal of the Ratitee—the true representative of the ‘‘ temporo- 
mastoid” of the Amphibia—sguamosal and preopercular in one, 
these structures show that the ancestors of the bird-kind were once 
on the lower Ichthyopsidan level. 
They could not, at that time, have been in a feathered stage; 
that form of covering cannot be imagined as clothing a kind of 
Tadpole; but a kind of Tadpole my have undergone metamor- 
phosis into a creature whose clothing was of feathers. 
The free edge of the perpendicular ethmoid (p.e.), behind the notch, 
has a convex outline above, and a concave outline below; the parasphe- 
noidal rostrum (pa.s.) (Plate XVII. figs. 1 and 3) projects forwards 
here as a sharp spike ; that grooved beam forms a common basis to 
the perpendicular ethmoid in front, and to the basisphenoid behind ; 
the presphenoid (p.s.) is tilted up above their junction, as in birds 
generally. The orbito-sphenoids (0.s.) are scarcely developed as 
distinct alee. The interorbital wall, made up of all these parts, is 
completely ossified and is moderately thick. ‘The orbital rim ends 
behind in a triangular postorbital process 5 millim. in extent ; it is 
over the notch leading to the moderately shallow, concave, temporal 
fossa (¢.f.), which is only 5 millim. from its fellow of the opposite 
side, and is 10 millim. broad below. 
1 Proc. Roy. Soc. 1888, pp. 394-402. 
