THE SKULL 



cated for them by the external nostrils. Conditions are not comparable 

 to those in the seals, for instance, but more like those in such mammals 

 as the tapir and moose (Alces), in which, in spite of the fact that the 

 nostrils are situated upon a prolonged proboscis, the nasal bones have 

 retreated far backward to allow for complicated musculature at the 

 base of the proboscis to give it the mobility required. The narial basin- 

 mg of sirenians — chiefly its broadening — is likely traceable to the same 

 stimulus, so that the muscles embracing the nasal passages may have 

 broader, firmer attachment upon its margins. To me these facts are 

 suggestive of the possibility that originally, in the sirenian ancestors, 

 external evidence of complicated nasal conditions may have been more 

 marked and that possibly they were equipped with a well developed 

 proboscis or comparable narial equipment. Strength is added to this 

 postulation not only by the fact that in the Eocene sirenian Eosiren, with 

 very long rostrum, the nasal basining extended to the posterior part of 

 the orbits, but also by the present sirenian equipment consisting of 

 highly specialized and mobile "lateral lips" and by the almost uni- 

 versally accepted belief that they are of proboscidean ancestry. The 

 conclusion suggested is that mammals of proboscidean derivation are 

 unusually amenable to the specialization of the narial and other facial 

 muscles in the production of probosceal or comparable equipment and 

 that the present result of this in the case of Sirenia is found in peculiar 

 labial specialization, and in the muscular conditions at the base of the 

 narial passages as indicated by the recession of nares and narial basining, 

 this being in part a relic suggestive of former and more specialized (in 

 one sense) nasal musculature. 



According as the trend of specialization be in one way or another 

 we might expect that eventually in Sirenia the nasal basining might either 

 be reduced in size by the recession backward of the suture between the 

 two premaxillae, and possibly by a narrowing as well, or else that it 

 might become shallower by a reduction in height of its bony borders, 

 and final broadening, the logical conclusion being a facial condition ap- 

 proximating in some respects that now occurring in the Odontoceti. 



Small, thickened, ovoid nasal bones at the side of the middorsal line 

 are present in Trkhechus\ there is no place for any such bones in Halt- 

 cove, and Hydrodamalis seems to occupy a middle position in this respect. 

 It is not known for a certainty that they were lacking in the latter genus, 

 for they would naturally have fallen from the weathered skulls that are 

 now available, and this cannot be settled by an examination of the speci- 

 mens. Some have considered that nasal bones were present but this I 



[in] 



