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Show 1874.] ANATOMY OF THE COLUMBAE. 253 is most probably the least. Next, with regard to the oil-gland, it is evident that genera such as Treron and Ptilonopus are not very far separated, notwithstanding that there are important differences between them; yet the former wants the oil-gland and the latter possesses it, though but feebly developed in P.jambu and P. maria, and absent (apparently) in P. melanocephalus. It is also known that among the Psittaci this structure may or m a y not be developed. These considerations lead m e to think that less stress should be laid on the oil-gland than on the two following characters. The constancy of the caeca, as a point of more than family importance in all other birds, would lead m e to consider their presence or absence as more significant than that of the oil-gland. In m y last communication to this Society* I gave reasons to show that the presence or absence of the ambiens muscle was a very significant fact in the classification of birds generally. This would lead me to lay considerable stress on the same point in any order or suborder in which it is found to vary. Assuming then, as in m y last paper, for reasons there stated, that the ancestral Pigeon possessed the ambiens muscle, and, on the same grounds, that it had caeca coli and an oil-gland, it is evident that the Pteroclidee, together with Columba, Turtur, Macropygia, and Ectopistes have departed least from the ancestral type. The Pteroclidee have branched off in another direction, as will be subsequently shown ; and therefore Columba, Turtur, Ectopistes, and Macropygia (together with those undissected genera unmistakably allied to them) may be considered to be the least modified, and therefore most typical of the Columbae. From these, if the peculiarities of the ambiens muscle have the importance which I assign them, a branch sprang, in which the ambiens was undeveloped. This includes at the present day Star-ncenas, Phlogoznas, Geopelia, Ptilonopus, Treron, and Goura, most of which possess 14 rectrices, and are confined to the Malay archipelago. This Treronine division, as it m a y be termed, seems to be preserved in its primitive form in Phlogoenas, in which no further structures are lost. Starnoznas, which, notwithstanding its peculiar distribution, must be considered as a member of it, loses the oil-gland, and Geopelia, as well as Ptilonopus, the caeca, whilst Treron and Goura are deficient in both. The main stem seems to have shortly given off a second branch, in which the caeca coli were alone wanting. This Phapine branch is now represented, without further complication, by Caloenas, Carpophaga, Chalcopelia, Chalcophaps, Chamapelia, Leptoptila, Leucosarcia, Metriopelia, Ocyphaps, Phaps, Tympanistria, and Zenaida; whilst from it has sprung Didunculus, without any oil-gland and with its quaint beak and remarkably long intestinal canal, which would indicate that its diet was usually one of fish, or more probably molluscaf. * P.Z. S. 1874, p. 111. t It is through the kindness of Prof. Newton that I have had the opportunity of dissecting a specimen of this rare bird. |