130: 



CLASS MAMMALIA. 



probably derived from Mammals with a heterodont dentition ; and 

 Professor Flower comes to the conclusion that it is most probable 

 that their ancestors were allied to the Ungulata ; while from the 

 absence of Cetacean remains in the Cretaceous the same authority 

 thinks that the earlier members of the order were inhabitants 

 of freshwater. With the exception of the Platanistidce and some 

 Delphinidce, all existing Cetaceans are of marine or estuarine 

 habits. 



In their increased number of phalangeals (hyperphalangism) the 

 Cetacea resemble the Reptilian IchthyosaiiridcB and Plesiosauridcz, and 

 from this circumstance, coupled with their simple type of teeth, it has 

 been argued that they represent the most archaic type of Mammals, and 

 even that they are directly descended from the Ichthyosaurs. As already 

 mentioned there are, however, many practically decisive objections to 

 these views. 



For the determination of fossil Cetaceans the solid tympanic and 

 petrosal bones of the internal ear, and less frequently the cranial 

 rostrum, are of especial importance, since these parts are generally 

 well preserved. 



Suborder i. Mystacoceti. — In this suborder, commonly known 

 as the Whalebone Whales, functional teeth are never present, al- 

 though germs may be developed 

 in the gums; " baleen" or whale- 

 bone is always attached to the 

 palate * the tympanic bone (fig. 

 1 1 72) is anchylosed to the peri- 

 otic and involuted upon itself; 

 the nasal passages are roofed in 

 by the nasals, the lachrymal is 

 small and distinct from the 

 jugal ; and the rami of the man- 

 dible are laterally curved, and 

 do not meet in a symphysis. 



Family Bal.enid^e. — The 

 only family of this suborder 

 may be divided into the Balce- 

 nine and Balanopterine sec- 

 tions. In the first of these there 

 is often no dorsal fin, the tympanic (fig. 11 72) has a characteristic 

 flattened and angulated shape, and some or all of the cervical ver- 

 tebrae are at least usually fused together. Remains of the genus 

 Balcena (including Balcenotus and Balcenuld) occur abundantly in 

 the Pliocene, and especially in the English and Belgian Crags. One 

 of these fossil species (B. affinis) is closely allied to the Greenland 

 Whale (fig. 1 1 73); while B. primigenia is more nearly related to 



Fig. 1 1 72. — Inner view of the right t3^mpanic 

 of the Greenland Whale (Balcena mystacetits). 

 One-third natural size. (After Gray.) 



