NATURAL SCIENCES OF PHILADELPHIA. 273 



of the present arrangement), that is, the depressed caudal vertebroe 

 and the development of transverse processes, has not bqen con- 

 firmed as to the Halitheriidae, and is applied on the supposition 

 that it is a co-ordinate of the other characters in which it approxi- 

 mates most to the dugongs. It must depend on future discovery 

 wliether tlie aggregate of common characters and the taxonomic 

 value of those characters confirm the present combinations. 



With respect to the Halicoridse and lihyfinidae, the conditions 

 of a natural sequence are apparently best fulfilled by the approxi- 

 mation of the former to the Halitheriidse. The Rlnjlinidse exhibits 

 a highly specialized modification in several features, and especially 

 in the absence of teeth and the length of the intestinal canal ; it, 

 however, affords a more generalized condition than the Halioridee 

 in other respects, and notably in the development of the carpal 

 bones, and cannot, therefore, b'e regarded as a derivative from the 

 Halitheridae family as now limited. But as it exhibits in all 

 respects a less generalized condition than the Halilheridse^ it must 

 be regarded as the result of development since the culmination oi 

 that type. The genealogical tree of the order may then be repre- 

 sented as follows : 



Proto-sirenian. 



(Halicoroidea) ( Trichechoidea) 



Trichechidoe 



Halitheriidce 



Halicoridae Rhytinidse. 



Various other extinct forms have been referred with more or 

 less reason to the order, but they are too little known to be intro- 

 duced with certainty into the system. 



