16 CHARLES F. W. McCLURE AND GEORGE S. HUNTINGTON 



14. Type CD, persistence of left supracardinal (C) and left 

 posterior cardinal veins (D). Left periureteric venous ring- 

 present, through which ureter passes. 



15. Type D, persistence of left posterior cardinal vein (D). 

 With the exception of Types ABD, ABCD, BCD, and CD, 



all the atypical forms above mentioned have been observed 

 in adult man or the cat either by us or by others. Type 

 ABCD, however, has been figured by Hochstetter ('93) as 

 occurring in Erinaceus europaeus. 



There are three additional types in which the vena cava 

 posterior may appear in the adult cat and in man in atypical 

 form: 



16. Persistence of embryonic supracardinal system of 

 veins to form a continuous system of vessels in the lumbar 

 and the thoracic regions, by means of which blood from the 

 iliac region can be conveyed directly to the heart, without 

 passing through the liver. 



17. Persistence of renal collar (circumaortic venous ring 

 at level of renal veins). 



18. Persistence of subaortic plexus or cardinal collateral 

 veins (C.C. in fig. 1). Marsupial Type of vena cava 

 posterior. 



Although for convenience of description we have classified 

 the potential variant or atypical conditions of the vena cava 

 posterior into the above-mentioned individual types, we rec- 

 ognize that in the same individual a combination of several 

 types does occasionally occur. Such a classification, based 

 upon a comprehensive ontogenetic plan of the veins, is a dis- 

 tinct advance over our former knowledge ; it not only permits 

 us to interpret the atypical forms of cava thus far observed, 

 but also enables us to predict others which are potential and 

 remain still to be found. 



