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Show 170 MR. W. K. PARKER ON STEATORNIS CARIPENSIS. [Apr. 2, large in the Raptorial Dicholophus, and also in the Procellariidae. In the Laridae it is smaller, and in the Alcidae (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 plana1. 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. pi. Iv. figs. 1,5, 6, and 13, t.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-quadrate " bar of the Ichthyopsida. In Steatornis the form of the " os uncinatum " (o.u.) is that of an inverted T: the stem is attached to the anteroinferior 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 Beptilian, 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 vertebrae of many birds, only let us down to the Reptilian level. But the " os uncinatum," the post-palatine, and the remarkable squamosal of the Ratitae-the true representative of the " temporo-mastoid" of the Amphibia-squamosal 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 m y have undergone metamorphosis 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. I 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 (o.s.) are scarcely developed as distinct alae. 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 (t.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. |
