THE ANATOMY OF THE COLUMBZ. 239 
the same direction as in the former case; and from it the four longi- 
tudinal muscular masses, which are here seen cut across, are well 
displayed, leaving a cruciform cavity between them, through which 
the food passes whilst being triturated. This gizzard is small in 
proportion to the size of the bird. No approach to a like condition is 
to be observed in Treron, the section of the gizzard in that genus being 
quite of the ordinary form figured above. 
It is generally said that the gall-bladder is absent in the Colambe ; 
and this is so in most of them; but besides being developed in the 
Pteroclide, it is found in all the species of Pitilopus, Lopholemus, and 
Carpophaga. In this point also Ptilopus therefore differs from Treron. 
The following Table contains the names of the different genera of 
the Columbe arranged in the manner suggested above. As a classi- 
fication of the suborder it is not at all my desire to put it forward as 
an ultimate one, but simply as the expression of the known facts of 
their structure. A more extended series of observations may indicate Page 258. 
reasons for modifying the stress laid on somé of the points; but it 
cannot, if my dissections are correct, change the facts themselves. 
As an instance of the direction in which alterations may be shown to 
be required, the case of the relation of Ptilopus and Treron may be 
taken; for, notwithstanding their general similarity in structure, these 
_ two genera present important points of difference. In the possession 
. of a gall-bladder by the former, it resembles Carpephaga; and it is 
quite possible that the similarity of the two genera under consider- 
ation is the result of similarity in habit only, and that whilst the 
latter (Treron) sprang from the Treronine stem direct, the former 
(Ptilopus) may have descended from the Phapine stock. This appear- 
ance of the gall-bladder in these genera is very difficult to explain, 
except on the supposition that in those in which it is absent it has 
been lost very lately, or that Carpophaga and Ptilopus are very 
intimately related forms. The uncertainty of its appearance in closely 
related genera of Mammalia reduces its value as a character of classi- 
ficational importance among them; but its very general absence in the 
Columbe makes it almost impossible not to lay considerable stress, in 
classification, on its presence. In the accompanying Table Ptilopus 
is placed in the same division with Treron because of their similarity 
in the points there laid stress on; but it requires a very inconsiderable 
amount more evidence to necessitate its being removed from there 
into close proximity with Carpophaga and Lopholemus. The fact that 
two forms in many respects so similar should differ so much in others 
is of itself a strong argument in favour of their separation; for the 
probability that forces which tend to produce marked external re- 
semblances should simultaneously develop internal differences is, to 
q say the least, extremely small. 
