1907.] AZYGOS VEINS TN MAMMALS. 213 



also, there are two azygos veins well developed. I have dissected 

 two examples, both females, of this species, and find that in both 

 the left azygos is rathei' the larger of the two, markedly so in one 

 specimen. This is a cnrious reversal of the prevalent arrangement 

 in Rodents and is reminiscent of conditions found among the 

 Marsupials. So far as my experience goes this condition of the 

 azygos veins is unique among the Rodents. It obviously culmi- 

 nates in the condition observable in the Beaver, where there is 

 but a single azygos A^ein and that of the left side. Precisely the 

 same series is to be met with among the Marsupials. 



(9) Tlie Azygos and other Thoracic Veins in the Young of 

 Myopotamus coypu, and the Homologies of the Azygos in Jlamtnals. 



A recent examination of foui- young specimens of the South 

 American Jlyopotamus coypu has furnished facts of gi-eat in- 

 terest in connection with the real nature of the azygos veins in 

 the Mammalia genei-ally, besides makmg a contribution to the 

 anatomy of these veins in that particular rodent. The four 

 individuals were born dead, but apparently were of full time. 

 They came into my hands on the following morning, and were in 

 admirable condition for study. All four agreed excepting in one 

 detail, and this apparent divergence may have been due to the 

 difficulty in tracing the smaller branches of veins. In all of these 

 examples there wei-e thi-ee veins in the thoracic region, as well 

 as — of course — the post-caval. Two veins (text-fig. 73) were 

 symmetrical with each othei-, and lay on either side of the ver- 

 tebral column near to the point of articulation of the ribs. Of 

 these veins the right-hand one was constantly the larger. Both 

 opened into the right and left anterior cava respectively. Branches 

 arising intercostally were seen in both cases. In front of the 

 point of entrance of this vein into the precava there were no 

 representatives of the venae intercostales suprenife. That is to say, 

 no veins to which this tei-m might be applied opened separately 

 into the precava. But as a matter of fact all the intercostal 

 spaces from the very first backwards had their intercostal veins, 

 which in the case of the anterior ones passed backwards to join 

 the main tru.nk. They joined the main trunk very nearly at its 

 point of entrance into the precaval vein, and the slightest shifting 

 of the posterior set of intercostals would divide the affluents of 

 the precaval into two veins — an anterior, corresjjonding to the 

 vena intercostalis suprema, and a posterior azygos. But is this 

 vein to be considered an azygos ? This point ma}- be deferred 

 until after description of the third thoracic vein. This vein was 

 rather larger than either of the paired veins just considered. 

 It lay in each case on the right side, or— to speak more accurately 

 — it poured its contents into the precava on the right and not 

 into the left-hand one of those veins, paired here as in many 

 other Rodents. The vein really lay rather medianly in position, 

 running over the centra of the vertebi-a?. Anteriorly it joined 



