218 MR. F. E. BEDDARD ON THE [Feb. 19, 



Broadly speaking, it appears to me that the trvie azygos is by no 

 means so variable a vein as are the persistent postcai'dinals, a 

 conclusion which is in pei-fect accord with general principles. A 

 partially persistent, though obviously decaying structure, is apt 

 to vary more than a newer structure, which may be increasing 

 rather than diminishing in importance. Thus we find that 

 the true right azygos is very constant among the Carnivora. 

 Secondly, there is the fact of the continuation into the abdo- 

 minal region even as far as the pelvic region of the azygos vein. 

 In some cases at any rate, possibly in the majority or even all 

 those animals which (e. g. Carnivora) possess a true azygos vein, 

 i. e. secondaiy vein not the persistent postcardinal, that vein is 

 continuous posteriorly with a longitudinal vein which receives 

 lumbar branches and opens indirectly or directly into the post- 

 ■caval vein. This part of it I regard, for reasons already put 

 forward, as a portion of the pei'sistent postcardinal. Now it is 

 precisely among the Diprotodont Marsupials, whose azygos veins 

 are, as I think, true postcardinals, that we find a very large vein 

 continuing into the abdominal and even pelvic region of the 

 body *". It seems to me that there is some significance in this 

 fact, and that it tends to support my contention that among 

 adult mammals the azygos veins fall into two categories. 



(10) The Condition of the Azijgos and the Classification 

 of Mammals. 



It is clear from the facts here set forth, that the condition of 

 the azygos vein or veins has a distinct relation to the mutual 

 afiinities of the several groups of Mammalia, but . in quite a 

 general way. Thus the Lemui'S agree with the other Primates in 

 having only the single right azygos ; and, moreover, it is in these 

 two groups alone that aii hemiazygos is at all prevalent on the 

 left side, joining the azygos some way before the latter opens into 

 the right vena cava superior. All the members of the Order 

 ■Cai-nivora agree with each other in the same character ; the 

 divergences in these three orders or two orders being of the 

 slightest when compared with some other groups. I do not 

 infer fi'om this that there is a special relationship between the 

 Carnivora and the Primates of course. Both groups have, as I 

 think, independently arrived at the same state of affairs as 

 regards the azygos vein. But for each group considered alone 

 the facts at least agree with the general view of their affinities. 

 It is important to note that the Lemurs agree with the Apes, 

 and the Arctoid Carnivora with the ^luroid. I^o difference in 

 mode of life seems to have aff"ected this deep-seated character. 

 The Otter cannot be distinguished in this particular from the 

 Civet Cat, or the Raccoon from the Suricate. 



* Hochstetter, however, declines to recognise as persistent sections cf post- 

 cardinal Robinson's description and tigures of a left postcaval extension of azygos 

 (Studies in Anat. Owens Coll. i. 1891, pi. vi. tigs. 1, 2, &c.). But it does not 

 follow that this objection would apply to Macropus. 



