466 ZOOLOGY SECT. 



of the head. The cervical region of the spinal column is very 

 short, and its vertebrae usually completely united together. 

 Clavicles are absent. The humerus is freely movable at the 

 shoulder, but all the other articulations of the limb are imperfect. 

 The number of phalanges of the second and third digits always 

 exceeds the number (three) normal in the Mammalia. The pelvis 

 is represented by a pair of horizontally placed styliform vestiges 

 of the ischia. Teeth may be absent and the mouth may be 

 furnished with sheets of baleen or " whalebone " ; when present 

 they may be very numerous and homodont, or less numerous 

 and heterodont, or reduced to a single pair. The epiglottis and 

 arytenoids are prolonged, and embraced by the soft palate, so as 

 to form a continuous tube for the passage of the air from the nasal 

 cavities to the trachea. The brain is large, and the cerebral 

 hemispheres are richly convoluted. The testes are abdominal. 

 The teats are two, and are posterior in position. The uterus is 

 two-horned, the placenta diffuse and non-deciduate (vide p. 576). 

 This order includes the Baleen Whales (Balcenidce), Sperm- 

 Whales (Physeter), Killers (Oca), Porpoises (Phoccena), and 

 Dolphins (Delphinus). 



Sub-order a. Mystacoceti. 



Cetacea in which plates of baleen are developed. Functional 

 teeth are never present, and the premaxillse are narrow and take 

 only a small share in the formation of the rostrum. The nostrils 

 are situated far back. The nasal cavities are roofed over by the 

 nasals. The tympanic bones are scroll-like, and are fused with the 

 periotics. The rami of the mandible are not united anteriorly. 



This sub-order includes the Whalebone Whales (Balcena and 

 others). 



Sub-order b. Odontoceti. 



Cetacea in which the premaxillae are narrow, and the nostrils far 

 back, as in the Mystacoceti. The nasals are reduced and do not roof 

 over the nasal cavities. The tympanic bones are not scroll-like, 

 and do not become fused with the periotic. The rami of the 

 mandible are united at the symphysis. Baleen plates are never 

 present, and teeth are developed, and are usually very numerous 

 and homodont. This sub-order comprises the Porpoises (Phoccena), 

 Dolphins (Delphinus and others), and Killers (Orca) ; the Sperm- 

 whales (Physeter and Cogia), the Bottle-nosed Whales (Hyperoodon), 

 and Beaked Whales (Mesoplodon). 



ORDER 3. SIRENIA. 



Aquatic Eutheria with moderate-sized head and fish-like, 

 depressed fusiform body, with the pectoral limbs paddle-like, the 



