260 MR. W. P. PYCRAFT ON THE MORPHOLOGY AND 
opercular fold grows backwards from the hyoid arch so as to cover the first and second 
branchial clefts. This has been noticed in some Neognathe. Sushkin [94] has figured 
and described it in Falco tinnunculus. ‘The first discovery of this operculum appears 
to have been made by Rathke [84] in 1832. 
PHYLOGENY OF THE PALHOGNATH. 
I enter upon the discussion of this subject with very considerable trepidation—not 
only on account of the difficulty of the problem, but also because it brings me into 
direct conflict with the opinions of those who speak after deliberation and long 
experience, and are, for this reason, and rightly, accounted authorities amongst us. 
It is now generally admitted that the Palwognathe must be descendants of an 
Avian proto-carinate form capable of flight. 
Parker [71] has given a number of characters which lend the strongest support to 
this view. ‘To many they will be convincing. The most important of these are :— 
“The fact that the skeleton of the fore limb is that of a true wing.” 
“The presence of an alar membrane or patagium.” 
“The presence of remiges and tectrices majores.” 7 
The alar membrane is a structure of great significance. According to Parker, it is 
the remnant of a once extensive scale-covered patagium. As these scales gradually 
became transformed into feathers the size of the patagium decreased. 
As to the mono- or polyphyletic origin of the group there is much divergence of 
opinion. 
Newton and Beddard consider the Palwognathe to be monophyletic; Fiirbringer, 
Gadow, and Parker polyphyletic. With these last I also agree, but not entirely so. 
There can be no doubt but that the Palwognathe represent a lower grade than the 
Neognathe. Nevertheless the latter are closely related thereto. 
It is probable that they have been derived as a diverging branch from that stock 
which gave rise to the Rheide, Dinornithide, and A%pyornithide. The diagram 
(fig. 10, p. 265) is intended to make this clear. 
The very close resemblance of the palate of the Crypturi to that of Rhea and 
Dinornithide and the general resemblance in the form of the pelvis of Crypturi 
and Dinornithide lend strong support to this view. 
The structure of the pelvic arch in the Palewognathe is of considerable interest, 
inasmuch as it bears upon the relation of this group to the Neognathe. 
It retains its most primitive form in Archwopterya, Dinornis, Apyornis, Hesperornis, 
Ichthyornis, Apteryx, and in the Crypturi. In all these the ischiadic fissure and free 
pubes are retained throughout life. In Meognathe the ischiadic fissure becomes 
converted into a foramen by synchondrosis between the ilium and ischium. In 
Struthio the ischiadic fissure is permanent, but the pubes fuse in the middle line, 
