1907.] AZYGOS VEINS IN MAMMALS. 219 



Again, the cavicorn Ruminants mostly agree with the Sviiclse in 

 the entrance of the left azygos into the heart independently of 

 the remains (if they exist) of the left vena cava anterior. And in 

 this Moschus and Hydropotes agree with them. This agreement 

 is not without interest, particularly when it is considered that the 

 Cervidaj may differ in the relations of the azygos : in Certms siJca 

 the right azygos being the prevalent or only azj^gos and entering 

 in the usual way the right anterior cava,. Most zoologists sepa- 

 rate the solid-horned from the hollow-horned Ruminaiits, though 

 to others the distinction between the two groups of Artiodactyles 

 appears " fanciful." In any case the Perissodactyles have their 

 own plan of azygos structure, which happens to agree with that 

 of some Cervidse, but distinguishes the Suborder from the Pigs 

 and hollow-horned Ruminants. In connection with the classifi- 

 cation of the Ungulates, as supported by the disposition of the 

 azygos veins, the position and number of these veins in Hy^^ax 

 are of particular interest. In this " subungulate," admittedly 

 primitive, and standing nearer to the base of the Ungulate series 

 than any other living form, except the Elephant, there is as it 

 were a hesitation to adopt definitely the form of the azygos veins 

 to be seen in either Artiodactyle or Perissodactyle. 



It may also be fairly said that a low position in the series is 

 indicated by the persistence of both azygos veins. These persist 

 in Echidna, and are very generally prevalent in the Marsupials, 

 the Rodentia, and, though here our knowledge is more deficient, 

 in the Insectivora. It is the general opinion that of these groups 

 at least three are primitive groups, and many would think the 

 same of the Rodentia. The division of the Marsupials into 

 Dipi-otodont and Polyprotodont is justified by the condition of 

 the azygos veins ; for in the Diprotodont division there is a much 

 greater tendency for the two azygos veins to persist than among 

 the Polyprotodonts. Furthermore, as emphasising the gap that 

 separates the Marsupials from most of the higher mammals, it is 

 noteworthy that in them (?'. e. the Marsupials) it is the left azygos 

 which is apt to be predominant and not the right vessel. In the 

 Order Rodentia also the conditions of the azygos agree with 

 current views as to their subdivisions. On the whole the 

 Hystricomorphine Rodents are those in which the two azygos 

 veins persist to a greater or less extent. An exception to this 

 statement would appear to be the genus Dipus. But it must be 

 remembered that the late Dr. Dobson concluded from certain 

 facts in muscular anatomy that Dipus was not remote from the 

 Hystricomorpha *. 



(11) The Position of the Azygos with reference to the Vertehrce. 



In dealing with the development of these veins in Mammals, 

 Hochstetter has dwelt upon the fact that the heart moves back- 

 wards and with it of coui'se the blood-vessels, including the 



* Joum. Anat. Phys. xvii. 1883, p. 177. 



