PULMONARY EVOLUTION IN MAMMALIA 133 
In view of the recent palaeontological evidence of the sepa- 
rate phylogenetic derivation of the Perissodactyl and Artiodactyl 
types, it is significant to note that the pulmonary extension, pre- 
sumably correlated with the same causative factors of body 
weight, muscular bulk and active locomotion, has been accom- 
plished in the two groups by different modifications of the 
respiratory pattern. The artiodactyl lung is characterized by 
the relatively enormous development of the cranial lobe of the 
right side, with the supplying right eparterial bronchus arising 
from the trachea above the bifurcation. In many forms this 
exaggerated development of the right cranial lobe amounts 
functionally to the introduction of a third lung, extending from 
the right side of the thorax across the median line and capping 
the apex of the left lung. As a matter of record the lung of the 
cetacean Pontoporia blainvillet has been described by Max Weber 
(36) as possessing a bronchial tree with triple division of the 
trachea into three main bronchi of unequal caliber. 
Among the artiodactyl ungulates the Camelidae (Camelus, 
Auchenia) and Giraffes are characterized by the unfolding of the 
typical artiodactyl right eparterial bronchus, arising from the 
trachea above the bifurcation, and in addition by the develop- 
ment of a left eparterial component from the stembronchus of 
the left lung. 
In the perissodactyls (with the exception of the Tapir), on the 
other hand, the demand for extension of the respiratory area is 
met by the bilateral development of the eparterial bronchus 
derived from the stembronchus of each lung. 
Among the aquatic mammalian forms the development of the 
lung in most cetaceans follows the artiodactyl type, while the 
pinnipede carnivores have developed the perissodactyl plan of 
pulmonary extension. It is noteworthy that the sole aquatic 
artiodactyl, Hippopotamus, agrees with the aquatic Carni- 
vores, in the bilaterally symmetrical development of the epar- 
terial component. 
The aquatic carnivore adaptations generally possess the power 
of rapid and active motion through the water, executed mainly 
by the highly developed axial and caudal trunk musculature. 
