xiii PHYLUM CHORDATA 451 



forwards. The teeth are heterodont, the anterior teeth being 

 simple and pointed, the posterior compressed, with two fangs and 

 denticulated cutting edges. 



This sub-order comprises only one known genus Zeuglodon- 

 of Tertiary age. 



Sub-order b. 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 Whale-bone Whales (Bahcna and 

 others). 



Sub-order c. 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 (Ddpliinus and others), and Killers (Orca), the Sperm- 

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

 and Beaked Whales (Mesoplodon), and the extinct Squalodonts. 



ORDER 3. SIRENIA. 



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

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

 pelvic absent, and with a horizontally expanded tail fin. There is 

 no vertical dorsal fin. There is a very thick wrinkled integument 

 devoid of or with only a scattered covering of hairs. The snout is 

 not greatly elongated, and the nostrils open by a pair of valvular 

 apertures on its upper surface. The cervical vertebrae (of which 

 there are only six in the Manatee) are not fused. A clavicle is 

 absent. There is a distinct, though small, articulation between the 

 humerus and the bones of the forearm. There are never more 

 than three phalanges in any of the digits. The pelvis is represented 

 by a pair of vertically situated vestiges. The anterior part of the 

 palate and the symphysis of the mandible (which is prolonged) are 

 covered with rugose horny plates. The epiglottis and arytenoids 

 are not prolonged as they are in the Cetacea. The brain is com- 

 paratively small, and the convolutions are not highly developed. 

 The testes are abdominal. The teats are two and pectoral in 



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