ART. 28 SKULL OF ZARHACHIS FLAGELLATOR KELLOGG 15 



but follows the groove on the ventral face of the attenuate process 

 of the orbitosphenoid in company with the trochlearis and the 

 ophthalmic division of the trigeminal. This groove is much wider 

 than in Inia, and that portion of the orbitosphenoid which separated 

 the optic canal from the above-mentioned groove has disappeared 

 on the skull of Lipotes. 



The observations of Anderson on the peculiarities of the orbito- 

 sphenoid in the skull of Platanista have been found to agree with 

 specimens in the United States National Museum. This descrip- 

 tion ^ can be consulted for a more detailed discussion. In Platanista, 

 the sphenoidal fissure is almost closed and the orbitosphenoid is 

 pierced by an extremely small foramen for the optic nerve. The 

 orbitosphenoid has been depressed and thrust backward so that its 

 posterior border slides backward above the anterior margin of the 

 alisphenoid. The ventral surface of the orbitosphenoid is broadly 

 and deeply grooved for the trochlearis and the ophthalmic division 

 of the trigeminal nerve which pass out through the sphenoidal 

 fissure and continue forward along this canallike continuation of the 

 fissure. As viewed through the foramen magnum the cranial orifice of 

 each optic canal is overhung by a thin platelike process of the orbito- 

 sphenoid. The minute ectal orifice of the orbitosphenoid portion 

 of the optic canal appears on the inner wall of the canallike continua- 

 tion of the sphenoidal fissure near the level of the anterior border 

 of the alisphenoid. The upper border of the external reduplication 

 of the pterygoid and a thin platelike process of the frontal which 

 slides forward under the maxilla, as viewed from in front, enclose the 

 passage for the optic nerve from below as it curves forward and 

 outward; the ectal orifice of the optic canal appears near the internal 

 extremity of the lachrymal. The optic canal, as it passes outward 

 along the supraorbital process, is bounded on three sides by the thin 

 plates of the frontal and roofed over by the maxilla. Tins tliin plate 

 like anteriorly directed process of the frontal also conceals the basal 

 half of the supraorbital process from a ventral view. The canallike 

 continuation of the sphenoidal fissure for the passage of nerves is 

 concealed by the pterygoid. Imperfections in the ossification of the 

 walls of the optic canal occur in most skulls of Platanista. 



In Zarhachis, the course of the optic nerve from the point where it 

 issues from the cranial cavity to the base of the supraorbital process 

 is concealed by the external reduplication of the pterygoid. The 

 edges of the bones surrounding the ectal orifice of the optic canal are 

 broken and jagged on the second skull and no attempt has been made 

 to remove the matrix from this region on the first skull because of its 



' Anderson, J., Anatomical and Zoological Researches: Comprising an account of the Zoological Results 

 of the Two Expeditions to Western Yunnan in 1868 and 1875. London (1878), pp. 511-512. 1879. 



