CHORDATA, 203 



Order 3. Cetacea. Marine (rarely fluviatile or estuarine) Mammals, 

 of fish-like form, with almost hairless skin, paddle-like fore-limbs with- 

 out nails, and horizontally flattened tail-fin. No hind-limbs, clavicles, 

 or distinct sacrum. Teeth, when present in adult, numerous, similar, 

 and without vertical successors. Placenta diffuse. 



(a.) Denticeti, toothed Cetaceans ; Zeuglodon (Tertiary). Delphinus 

 delphis, dolphin. Phocsena communis, porpoise. Monodon, narwhal. 

 Physeter tursio. 



(6) Mysticeti, right whales. Balsenoptera rostrata. Balsena Mys- 

 ticetus. 



Order 4. Sirenia, sea-cows. Aquatic Mammals with scanty hair, 

 paddle-like fore-limbs with rudimentary nails, and horizontally flattened 

 tail-fin. No hind-limbs, clavicles, or sacrum. Dentition herbivorous. 

 Placenta diffuse. Manatus, manatee (E. coast of S. America and W. 

 coast of Africa), with six cervical vertebrae. Halicore, clugong (shores 

 of Indian Ocean and Red Sea). Rhytina Stelleri, exterminated last 

 century. Halitherium, Pliocene. 



