10 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 67 



The posterior face of the Zarhachis skull resembles Lipotes more 

 closely than Inia. The supraoccipital is wider than high, bounded 

 on the sides and at the top by a flaring lambdoid crest. In Lipotes, the 

 supraoccipital is traversed by a distinct median ridge, extending from 

 the lambdoid crest almost to the foramen magnum. A low median 

 crest is present on the upper half of the supraoccipital in Zarhachis. 



The unusual depth of the thin lambdoid crest is the characteristic 

 peculiarity of the back of the skull. On the inner and lower sides of 

 the temporal fossa the crest curves ventrally and laterally, following 

 the contour of the posterior border of the fossa. At the level of the 

 lower border of the temporal fossa, the crest on the right side is fully 

 32 mm. deep. The thin lambdoid crest overhangs the exoccipital on 

 each side. The exoccipitals project downward and backward, and 

 are of a different shape than those of Lipotes and Inia, the most no- 

 ticeable modification in the latter genera being associated with the 

 development of a paroccipital process. In these living river dolphins, 

 the exoccipital is distinctly constricted above the paroccipital process. 

 This process projects outward and backward. In Zarhachis, the ex- 

 occipitals are larger, although they are not produced outward far 

 enough to conceal the zygomatic processes from behind. The more 

 detailed features of the exoccipitals are shown on the photograph re- 

 produced on Plate 3. Laterally and anteriorly, the exoccipital is in 

 contact with the squamosal, superiorly it is coalesced with the supra- 

 occipital, while below and internally it fuses with the basioccipital. 

 The deep jugular incisure may mark the junction of the exoccipital 

 with the falcate process of the basioccipital. 



The foramen magnum originally was probably slightly wider than 

 high. The condyles are seraielliptical in outline. They are strongly 

 convex from side to side and are borne on distinct necks. The 

 internal borders of the condyles are sharp edged and concave; the 

 external borders are rounded off. 



Lateral view. — Aside from the relatively large size of the " upended " 

 extremity of the supraorbital process and the massive zygomatic 

 process, the skull (pi. 4) is characterized by a short temporal fossa 

 and a protuberance on the vertex. The protuberance behind the 

 nasal passages is the highest point on the dorsal profile. The trans- 

 verse crest of the supraoccipital is the next highest point and from 

 this crest the maxilla slopes forward to the base of the rostrum. 



Above the orbit the extremity of the supraorbital process bends 

 abruptly upward and forms a broad crest. Portions of the outer 

 margin of the maxilla are visible on the upper margin of this crest 

 from a side view and it also sheathes the entire internal surface. 

 The greatest vertical depth of the right supraorbital process is 64 

 mm. and the, greatest anteroposterior diameter is 96 mm. The 

 external face of the extremity of the supraorbital process is more or 



