NATURAL SCIENCES OF PHILADELPHIA. 267 



Certain Zeuglodonts are equall}^ characterized by the absence of 

 a rostrum, and tlie position of the narial aperture. 



The Denticete alone are characterized by the absence of olfac- 

 tory nerves, and the rudimentary condition of the nasal bones. 



The Mysticete only are distinguished by the loose connection 

 of the rami of the lower jaw at the symphysis. 



c. Those not differentiating from the Cetaceans, or even from 

 sub-orders, but onlj' from minor types, viz., the non-consolidation 

 of cervical vertebroe, the smaller number of vertebne (50 to 53 

 against " at least" 60), the articulation of the ribs, the absence of 

 asymmetry of the bones on each side of the head, and the presence 

 of a lachrj^mal bone. 



The Iniidse, Platanistidae, and others, have as completel3'^ sepa- 

 rated cervical vertebrae as the Manatidae. 



The number of vertebra is less in Inia (41), Pontoporia (42), 

 Anarnacus or Hyperoodon (44 to 45), and others, and is, at the 

 same time, of very slight taxonomic value. 



The articulation of the ribs is only of family value. 



The asymmetry of the cranial bones is most evident in the 

 Plnjseteridse^ very slight or almost none in many Denticete, and 

 is not greater in the Mysticete than in the Sirenians. 



The lachrymal bone is evident in the family'' of Ziphiids, and in 

 the Mysticete. 



Of course, then, onl}' the characters differentiating the Sirenians 

 as a whole from the Cetaceans as a whole can be considered as 

 diagnostic or determinative of differences. Tlie other alleged dis- 

 tinctions decrease in value in ratio to their restriction to subordi- 

 nate groups. 



But in addition to the characters alleged to differentiate the 

 Sirenians from the Cetaceans, others may be found in the modi- 

 fications of the brain, heart, etc., and all together ampl}'^ support 

 the validity of the distinction of the ordinal separation of the 

 Sirenians from the Cetaceans, but they do not prove the propriety 

 of their ordinal union with an}'' Ungulates. This must be proved 

 by the discovery of at least one character exclusivel}^ common to 

 the Sirenians and Ungulates ; we now propose to inquire if there 

 be such. 



2d. Characters common and confined to the Sirenians and the 

 Ungulate animals^ or to any of the subordinate groups of Ungulates.. 



These may be examined under several heads. 



