408 CLASSIFICATION OF VERTEBRATES. 



bladder is present. The larynx is prolonged so that it enters 

 the choana, a condition recalling the marsupials. The testes 

 are abdominal, the uterus two-horned, and the two mammae are 

 in grooves near the vulva. These are provided with constrictor 

 muscles, by which the milk is forced into the mouth of the 

 young. About two hundred existing species are known. 



The cetacea are introduced by the zeuglodons in the eocene, 

 the other groups appearing in the miocene. 



SUB-ORDER i. ARCH^EOCETI. 



External nostrils at the middle of the muzzle ; nasals long ; temporal 

 fossa elongate ; ribs bicipital ; anterior teeth with single roots, posterior with 

 two roots, the free edges dentulate. Breast bone of several sternebrae. 

 Cervicle vertebrae free. 



Zeuglodon {Basilosaurus) the only genus (with several subgenera) comes 

 from the eocene of our southern states, Europe, and New Zealand. One 

 species was 60 feet long. 



SUB-ORDER 2. ODONTOCETI (DELPHINOIDEA, DENTICETVE). 



Skull asymmetrical ; external nares united at base of snout; nasals very 

 small ; temporal fossa short ; teeth present in both jaws or only in lower, 

 occasionally reduced to a single pair. Olfactory organs absent or rudimen- 

 tary ; anterior ribs bicipital, others with only a tubercular head ; sternum of 

 several sternebrae. 



The toothed whales are all carnivorous, but much as they have been 

 pursued by man, many questions concerning the features presented by any 

 species at different ages are still unsettled. The SQUALODONTID.E, with 

 teeth in both jaws, and these differentiated into incisors, canines, and two 

 kinds of molars, occur in the miocene and pliocene. In some respects they 

 seem intermediate between the zeuglodons and the toothed whales, but 

 their skeleton is imperfectly known. The PLATANISTID.E include the fresh- 

 water dolphins, Platanista and Inta, from the Ganges and Amazons ; allied 

 forms occur in the miocene and pliocene of both continents. The true dol- 

 phins (DELPHINID^E) have the snout elongate, no premaxillary teeth, the other 

 teeth variable, usually conical, and with a single root. Teeth are numerous 

 in both jaws of the true dolphins and porpoises {Delphinus^ Tnrsiops, Pho- 

 ccena, Neomeris) ; are fewer in the black fish (Globiocephalus) and the killer 

 whales (Orca\ these latter being the wolves of the sea. The white whale of 

 the Arctic seas is Delphinaptenis, which in all points of structure except the 

 numerous teeth is closely allied to the narwal {Monodon}, in which, in the fe- 

 male, all the teeth are functionless, while in the male one left maxillary tooth is 

 developed into an enormous spirally twisted tusk, which sticks straight out from 

 the head. Its functions are unknown. Several of these genera, the narwal in- 

 cluded, occur as fossils in the later tertiary and pleistocene. The PHVSETER- 



