S86 MAMMALS. 



intervals than in terrestrial mammals. The water vapour 

 expelled along with the air from the lungs, condenses into 

 a cloud, which is sometimes increased by an accidental 

 puff of spray. 



The kidneys are lobulated. The testes are abdominal. 

 There are no seminal vesicles. The uterus is two-horned. 

 The placenta is non-deciduate and diffuse. The two 

 mammffi lie in depressions beside the genital aperture, 

 and the milk is squeezed from special reservoirs into the 

 mouth of the young. Usually a single young one is born 

 at a time, and there are never more than two. 



All are carnivorous, but while many feed on small pelagic 

 animals, others swallow cuttles and fish, and Orca attacks 

 other Cetaceans and seals. Most are gregarious and live in 

 schools or herds. 



The living Cetaceans are ranked in two sub-orders — the 

 Mystacoceti or Balsenoidea without teeth but with whale- 

 bone or baleen plates on the palate, and the Odontoceti or 

 Delphinoidea, with teeth and without baleen. 



Some Eocene fossils referred to the genus Zeuglodon, are 

 regarded by some as representative of an extinct sub-order 

 — Archaeoceti — but D'Arcy Thompson has advanced strong 

 arguments in favour of their affinities with Pinniped Car- 

 nivores. 



In regard to the possible affinities of the Cetacea, Flower 

 maintains (i) that the hypothesis of their descent from 

 Ichthyopterygian reptiles is untenable, (2) that they are 

 separated from an alliance with Carnivora by many essential 

 characters, (3) that they exhibit several, though by no means 

 close, affinities with Ungulata. 



The same authority refers to several facts which suggest 

 that, in their transition from terrestrial to marine life, the 

 Cetaceans may have passed through a stage in which they 

 lived in fresh water. 



