10 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 63, 



Piaz because the extremities of the supraorbital processes are missing 

 and the cranium is very imperfectly preserved. 



Whenever forms which are manifestly different from each other 

 as regards certain structures are associated together within limited 

 groups, it follows that such forms should possess some fundamental 

 structures in common. Here again difficulties are obvious for each 

 investigator naturally holds that the structures with which he is most 

 familiar are fundamental. Usually, it is merelj^ a question of the 

 relative importance to be attributed to each set of structural pecu- 

 , liarities and what gi'oup or groups of forms will best elucidate the 

 particular features which each investigator considers most important. 

 In the opinion of the writer, fundamental structures are to be found 

 in the periotic and tympanic bones of the Cetacea. In view of the 

 peculiar combinations of characters which are found in the above- 

 mentioned genera and the general similarity of the earbones, it ap- 

 pears that Zarhachis represents one line, Platanista a second line, and 

 Inia or Lipotes a third line of a common ancestral stock. 



Dorsal view. — The general arrangement of the elements compris- 

 ing the dorsal portion of this fossil skull (pi. 1) is similar to that of 

 Lipotes vexillifer.^ The differential characters of the present species 

 are shown by the extremely long attenuate rostrum and the accom- 

 panjdng elongation of the ankylosed symphj^sis of the mandibles, 

 the prolongation of the zygomatic process of the squamosal and of 

 the postorbital portion of the supraorbital process of the frontal so 

 that there is actual contact between them, and the absence of an 

 elevated vertex. 



The long, flattened, and attenuated rostrum comprises more than 

 five-sixths of the total length of the skull and is neither bowed up- 

 ward nor bent downward. Anterior to the premaxillary foramina, 

 the premaxillse are thick and convex; they decrease in width and in 

 height toward the tip of the rostrum. If the homologies of the bones 

 forming the tip of the rostrum are correctly understood, then the 

 premaxilla extends forward beyond the maxilla as a wedge-shaped 

 splint which is closely appressed to the large recurved front tooth. 

 A small V-shaped indentation (15 mm. long and 6.5 mm. broad 

 anteriorly) separates the two premaxillae at the tip of the rostrum; 

 this indentation leads to a small canal which presumably represents 

 the roofed over mesorostral gutter. 



The inner margins of the premaxillae become closely appressed to 

 one another at a point 165 mm. in front of the maxillary notches 



^ Miller, G. S., Jr., A new river dolphin from China. Smiths. Misc. Coll., vol. 68. 

 No. 9, Publ. 2486, pp. 1-11, pis. 2, 4, 6, Washington, 1918. 



