CLA SSIFICA TION 



97 



have been. There is, therefore, obviously some diffi- 

 culty in ascertaining, or endeavouring to ascertain, 

 what are the real differential characters of the group ; 

 to separate, that is to. say, characters due to the 

 environment and those which have been inherited 

 from the long extinct terrestrial ancestor. The 

 current definitions of the group Cetacea are obliged 

 to be founded on these, as we must assume them to 

 be, recently -acquired characters. To take one or 

 two as examples. 



Professor Zittel* defines them in the following 

 terms: "Naked, smooth - skinned, fish-like water- 

 dwellers, with cylindrical body. Head not separable 

 from the body. Nasal orifices on the upper side lying 

 far back. Anterior limbs fin-like, hind limbs wanting. 

 Tail fin horizontal. Milk glands abdominal in position." 



Messrs. Parker and Haswellt use the following 



o 



language : ''Aquatic Eutheria, with large head, fish- 

 like, fusiform body, devoid of hairy covering, with 

 the pectoral limbs paddle-like, the pelvic limbs absent, 

 and with a horizontal caudal fin. A vertical dorsal 

 fin is usually present. There is a long snout, and 

 the nostrils open by two lateral external apertures 

 or a single median one, situated in all recent forms 

 far back towards the summit 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 



* Handbuck der Palceontologie, iv., p. 155. 

 Text-book of Zoology, vol. ii., p. 450. 



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