170 MR. W. K. PARKER ON STEATORNIS CARIPENSIS. [Apr. 2, 



large in the Raptorial Dicholophus, and also in the Procellariidse. 

 In the Laridse it is smaller, and in the Alcidse {Alca torda, TJria 

 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. 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-quad- 

 rate " bar of the Ichthyopsida. In Steatornis the form of the " os 

 uncinatum " (o.m.) 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 hones, seen in the Tinamous and some other binds, the 

 sutures in the skulls of those Gallo-struthious birds, and the opis- 

 thocoelian 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 Ratitse— the true representative of the " teniporo- 

 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 ?i 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 {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 {tf.), which is only .5 millim. from its fellow of the opposite 

 side, and is 10 millim. broad below. 



1 Proc. Eoy. Soc. 1888, pp. 394-402. 



