14 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 6T 



united with the exoccipital along its external border. To the inside 

 of the postglenoid process and at the origin of the groove for the 

 external auditory meatus, the periotic was attached to the skull. A 

 thin plate-like process of the squamosal which is directed inward and 

 downward contributes the external wall of the tympano-periotic fossa; 

 the internal wall is formed by the falcate process of the basioccipital. 

 Between the glenoid process and the internal margin of the glenoid 

 fossa there is a sharply defined longitudinal depression, which curves 

 forward from the base of the groove for the external auditory meatus 

 to the anterior margin of the squamosal. The contour of the ventral 

 surface of the parocci]>ital process resembles that of Inia and Pla- 

 tanista. There are some minor modifications which are not developed 

 on the paroccipital processes in the last-mentioned genera and of these 

 the thin crest on the anterior border is the most conspicuous. 



Further comj)arisons with skulls of the living Iniidae and with 

 Platanista show some interesting structural modifications in the 

 region of the orbitosphenoid. In Inia, the sphenoidal fissure is rela- 

 tively large, bell-shaped in contour, and bounded laterally and on 

 the rear by the basisphenoid, and anteriorly by the orbitosphenoid. 

 On their outward course, the trochlearis and the ophthalmic division 

 of tlie trigeminal nerve follow the groove on the ventral surface of 

 an attenuate process of the orbitosphenoid. This groove is rather 

 deep, with Avell-marked lateral walls; the process as a whole is directed 

 obliquely outward and forward, terminating near the base of the 

 broad channel on the supraorbital process of the frontal. The most 

 remarkable peculiarity of the orbitosphenoid of Inia is the position 

 of the ectal orifice of the optic canal. This orifice appears on the 

 inner wall and about halfway between the anterior and posterior 

 ends of the groove leading forward from the sphenoidal fissure. 

 From this point forward all three nerves occupy the same groove. 

 Within the cranial cavity, the orifice of each optic canal is placed 

 near the inner angle of the sphenoidal fissure and is overhung by a 

 thin platelike process of the orbitosphenoid. The course of the optic 

 canal through the orbitosphenoid parallels the groove leading forward 

 from the sphenoidal fissure. The apex of the process of the orbito- 

 sphenoid which contributes the surface of the groove for these nerves 

 is jagged, and projects outward and forward at a lower level than the 

 ventral surface of the supraorbital process. 



In Lipotes, the sphenoidal fissure is narrower, bounded on the 

 rear and on the outside by the alisphenoid, and on the inside and 

 in front by the orbitosphenoid. The orbitosphenoid of Lipotes has 

 the same general shape as that of Inia and is likewise characterized 

 by a narrow attenuate process which is grooved for the passage of 

 nerves. The course of the optic nerve is slightly modified in com- 

 parison with Inia. This nerve does not pierce the orbitosphenoid, 



