222 ON THE AZYGOS VEINS IN MAMMALS. [Feb. 19, 



each precaval has its own separately debouching intercostals. 

 That this is not at all due to the prevalence among these Rodents 

 of two precavals is shown by the same constancy of the vena^ 

 intercostales supremee among the Artiodactyle Ungulates. The 

 exceptions that I have met with to the rule that these veins are 

 present on both sides and open into the single precaval are but 

 few in number. Concerning other groups the facts at my dis- 

 posal are so few that I hesitate to attempt any i-emarks of a 

 general character. 



(13) Conclusions. 



Although the facts detailed in the preceding pages, as well as 

 those made known by the records of others, might be largely 

 added to, enough appears to me to be known to permit of a few 

 general observations upon the azygos vein in the Mammalia. 



(1) With the exception of the Cetacea the azygos is always a 

 well-developed vessel of fair size among the Mammalia, and 

 generally to be found fully de^■eloped upon one side of the body 

 only, and this the right. Its functions appeal- never to be taken 

 up by the vena cava inferior, so that it is never rudimentary. 



(2) There is only a right azygos with no traces of a left — 

 except rarely as a hemiazygos not reaching the vena cava anterior 

 — in Carnivora, Lemurs, American Edentates, and Primates. 



(3) The existence of two azygos veins characterises several 

 Marsvipials, Rodents, Insectivora, and most Artiodactyles. In the 

 two first-named groups the existence of the two veins is associated 

 with the presence of two superior venfe cavpe. It is not, however, 

 every Rodent with two superior cavse which has also two venfe 

 azygos. But, as it appeaivs, no Rodent with one superior cava 

 only has more than one vena azygos. The Artiodactyles are 

 exceptional in that, while possessing only a single anterior vena 

 cava, there are two azygos veins. But the left azygos generally 

 opens into the right auricle with or close to the vena cava posterioi-. 

 Rarely there is also a connection with the single vena cava 

 anterior. Where two azygos veins are present it is unusual for 

 them to be equally developed. Either the right or the left is 

 well developed and the other smaller or much reduced. 



(4) The existence of a left azygos only characterises a few 

 forms belonging to the groups Marsupials, Rodents, and Artio- 

 dactyles. This condition of the azygos is not, like the two which 

 have been dealt with, characteristic of any large group. 



(5) It is to be remarked that, on the whole, the possession of 

 two azygos veins is associated with a low position. 



(6) With certain exceptions (e. g. ^Epi/pri/7n7ius riifescens) the 

 azygos vein is fairly constant in its chai-acteristics for a given 

 species. This is sometimes very markedly the case. 



(7) As will be gathered from the details given, the azygos is 

 of some use in the classification of Mammals. Thus Hyrax shows 

 its Ungulate affinities, and several genera of Hystiiciform Rodents 

 agree together in the character of the azygos veins, &c. 



