1907.] AZYGOS VEINS IN MAMMALS. 207 



Dr. McClure is of opinion that "a single azygos vein is the rule 

 in Marsupials, and that when two are present the case may be 

 regarded as a variation." The facts gathered by myself seem to 

 me to show that the double azygos of the Monotremata is largely 

 preserved in the Marsupials, so much so that the process of dis- 

 appearance of one or the other is but rarely completed. There are 

 but few forms in which there is absolutely no vestige of either 

 right or left azygos as the case may be. This is precisely what 

 would be expected in view of the admittedly archaic position of 

 the Ma.rsupials, accepting, of course, the view now generally held 

 that they are not intermediate between the Prototheria and the 

 Placentals, but are an offshoot of an early Eutherian. For in 

 the Rodents, an admittedly primitive type, we have considerable 

 traces of the double azygos. This latter point, however, will be 

 discussed in relation with the azygos in Mammalia generally on 

 a subsequent page. On the other hand, my own observations 

 confirm Dr. McClure when he remarks that a right azygos is 

 characteristic of the genus Ilacrojius, and that a left azygos is 

 characteristic of the Phalangerid?e. 



I would rather extend Dr. McClure's remarks about the condition 

 of the azygos vein in the Carnivorous Marsupials, and point out 

 that in this whole group the existence of a prevalent left azygos 

 is the rule so far as we know at present. The table given by 

 Dr. McClure of the condition of the azygos in such Marsupials as 

 were known when he wrote, brings out very clearly the variation 

 in respect of the veins not merely from genus to genus but from 

 species to species. This generalisation I confirm with great con- 

 fidence, and also would emphasise the very frequent variation of 

 these veins from individual to individual. The condition of the 

 azygos in fact is by no means so fixed in this group as it is in the 

 Carnivora, for example ; and modification is evidently progressing 

 along two lines, in one of which the right azygos and in the other 

 the left azygos is being retained, while the opposite vein is in 

 process of disappearance. 



jSToteworthy, too, is the occasional large size of the postdia- 

 phragmatic continuation of the azygos in the Kangaroos, Macropus 

 giganteits, M. agilis, &c., which is, so far as my own observations 

 go, invariably of the light a,zygos. 



The above results may be conveniently tabulated as follows : — ■ 



a. Azygos on both sides equal or nearly equal. 



Macro2nis tialahatus M. hruni, II. derhianus, M. hennettii, 

 Fliascolomys mitchelli, Bettongia ogilhyi, JE2yypry'mnus rufescens, 

 Thylacinios cynocephcdus, Dasyurus viverrinus. 



h. Azygos large on right side, with a rudiment on left of not 

 more than three intercostal affluents. 



Macropus dorsalis, M. giganteus, M. rufus, M. cdligatoris, 

 M. antilo2nnus, M. tnelanops^ Petrogcde 2>enicillata, Bettongia peni- 

 cillata. 



