260 ME. W. P. PYCEAPT ON THE MOEPHOLOGY AND 



opercular fold grows backwards from the hyoicl arch so as to cover the first and second 

 branchial clefts. This has been noticed in some Neognathce. Sushkin [94] has figured 

 and described it in Falco tiwnunculus. The first discovery of this operculum appears 

 to have been made by Rathke [84] in 1832. 



PHYLOGENY OF THE PAL^OGNATH^. 



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 Palceognathce 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." 



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 Palceognathce 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 Palceognathce represent a lower grade than the 

 Weognathce. 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 Bheidce, Dinornithidce, and JEpyornithidce. 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 

 Dinornithidce and the general resemblance in the form of the pelvis of Crypturi 

 and Dinornithidoe lend strong support to this view. 



The structure of the pelvic arch in the Palceognathce is of considerable interest, 

 inasmuch as it bears upon the relation of this group to the Neognathai. 



It retains its most primitive form in Arclmopteryoc, Dinomis, ^Epyornis, Hesperomis, 

 Ichthyomis, Apterycc, and in the Crypturi. In all these the ischiadic fissure and free 

 pubes are retained throughout life. In Neognathce 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. 



