12 PROCEEDINGS OF THE NATIONAL MUSEUM vol.67 



It was stated in the preceding paper that the outer plate of the 

 pterygoid represented the external pterygoid and the internal plate 

 was referred to as the internal pterygoid. This interpretation does 

 not appear to be correct, since both plates are continuous on the right 

 side of the second skull and overspread the inferior face of the basal 

 portion of the alisphenoid ; these combined plates and the thin isthmus 

 which unites them form the walls of the longitudinal sinus external to 

 the basisphenoid. In view of the above, it is apparent that the outer 

 plate represents the external reduplication of the pterygoid. The 

 loss of the anterior end of the thin platelike external reduplication of 

 the pterygoid on the right side exposes the pyramidal cavity on the 

 corresponding side of the axial ridge of the vomer for its entire length. 

 On the left side, this thin platelike external reduplication of the ptery- 

 goid extends forward 128 mm. in advance of the posterior wall of the 

 nasal passage and contributes the external wall of the pyramidal 

 cavity on the left side of the vomer. Posteriorly, the external re- 

 duplication of the left pterygoid extends backward beyond the nasal 

 passages and is bounded by the maxilla anteriorly, by the frontal 

 and alisphenoid superiorly, and by the squamosal posteriorly. The 

 mandibular branch oi the trigeminal nerve passes outward through 

 the foramen ovale in the suture between the squamosal and the ex- 

 ternal reduplication of the pterygoid and foUows the trowellike gutter 

 on the external face of the latter. This foramen is situated in ap- 

 proximately the same position as in Platanista. There is a remark- 

 able resemblance between the size and relations of these pterygoids 

 and the corresponding bones in Platanista. The internal plates of the 

 pterygoids are not as well preserved on this specimen as on the first 

 skull. On the latter the thin internal plate of the pterygoid curves 

 around the lower border of the lateral and anterior walls of the cor- 

 responding nasal passage and meets the vomer mesially; it then 

 turns almost at right angles to the latter and is closely appressed to 

 the external surface of the trough of the vomer. On skulls of Inia, 

 Lipotes, and Platanista thin plates of the maxillae have overspread 

 the external surface of the trough of the vomer and conceal all but 

 the axial ridge of the latter. On the ZarhacMs skull the external 

 surface of the trough of the vomer is not overspread by the maxillae 

 below the level of the palatines. Further comparison with the first 

 skull is scarcely necessary, as it is now apparent that the thin an- 

 teriorly directed internal plate of the pterygoid did not by itself 

 form the internal and superior walls of the pyramidal cavity or at 

 least a cavity was not formed which was bounded solely by the in- 

 ternal plate of the pterygoid and its external reduplication as in 

 Platanista. 



The palatines are not visible from a ventral view when the ptery- 

 goids are complete. The sutures defining the limits of the right 



