1907,] AZYGOS VEINS IN MAMMALS. 205 



branches entered either of the two precavals. The light azygos 

 entei'ed the precaval at about on a level with the fourth lib. The 

 first branch arose between the third and fourth ribs. The position 

 of the opposite azygos was the same. The right azygos continued 

 down to the tentli rib unaltered ; at this point it divided into two 

 branches, of which the outer supplied the intercostal spaces ; the 

 inner branch, which was more slender, received at least one cross 

 branch from the outer and could be traced some little way back as 

 a very slender vessel ; I did not ascertain its posterior connections 

 if any. Just after its bifurcation it was crossed by the first inter- 

 costal artery to cross it, those in front lying below the azygos. 

 This division of the azygos posteriorly reminds us of the figure of 

 the Pig's azygos given by Messrs. Parker and Tozier*. But it 

 seems more likely that the inner branch is not the hemiazygos, 

 but possibly the subcardinal of the right side, the outer branch 

 being in that case the true persistent postcardinal. These matters, 

 however, are more fully dealt with below. The left azygos of this 

 example of Phascolomys mitchelU was shorter than the right ; it 

 ended absolutely between the tenth and eleventh ribs. 



In the Brush-tailed Bock- Wallaby {Peirogcde jjenicillata) the 

 disposition of the azygos veins was like that which is on the 

 whole characteristic of the genus Macropus. I have dissected 

 three examples, one male and two females. They all agreed 

 except in minutife. In all of them the right-hand azygos was the 

 one to be well developed. But I never found that this vessel had 

 a prominent backward prolongation to the ^Jelvic region as is 

 occasionally to be seen in Macropus. On the left side there is 

 only a trace of the azygos, and this is either derived from only 

 one or from two intercostal twigs. In F. xanthopus, according to 

 Parsons t, the right azygos is also the larger, or rather the only 

 one present. 



I do not know whether it is necessary to separate Bettongia 

 peniciUata from B. ogilhyi. The condition of the azygos veins 

 ofiers no help in deciding this question, since the two examjDles of 

 B. 2)enicillata differ considerably from each other, and I have only 

 a single example which came into my hands labelled Bettongia 

 ogilhyi. In one specimen of Bettongia j^^it-icillata, a female, the 

 right azygos alone was well developed, extending far back towards 

 or to the diaphragm. On the left side the azygos was very short, 

 consisting of two intercostal affluents only. In the second example 

 of this species, which was also a female, the azygos on both sides of 

 the body was about equally developed. In the single Bettongia 

 ogilhyi the two azygos veins were well developed ; but that at the left 

 side was distinctly the longer of the two. It is to be noted that if 

 we are to unite these species, as Mr. Thomas has done in his 

 ' Catalogue of Marsupials in the British Museum,' the differences 

 exhibited in respect of the azygos vein are almost exactly the same 

 as those shown by u:'Epypry']nnus rvfescens. 



* Bull. Mus. Comp. Zool. xxxi. p. 138, fig. 4. 

 t See P. Z. S. 1896, p. 706. 



