22 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.63, 



bral orifice of the facial canal. The position of the foramen singu- 

 lare corresponds more closely with Inia than with Platanista. In 

 the latter, the foramen singulare is present as a minnte opening on 

 the posterior wall of the facial canal. In the periotic of this fossil 

 porpoise, the foramen singulare is situated relatively nearer to the 

 spiral tract although a low partition separates these structures. The 

 cerebral orifice of the aquaeductus vestihuli is of moderate size and 

 elongate; the orifice is situated external and slightly posterior to 

 the internal acoustic meatus as in Platanista. A narrow isthmus of 

 bone lies between the cerebral orifice of the aquaeductus cochleae and 

 the fenestra rotundum; the canal is relatively large. In both Inia 

 and Platanista the aquaeductus cochleae and its cerebral orifice are 

 very minute. The cerebral orifice of the aquaeduct of the cochlea 

 in this fossil periotic is situated 2.6 mm. from the internal acoustic 

 meatus and at least 4.5 mm. from the same orifice of the aquaeduc- 

 tus vestihuli. The 'pars vestibula7is is relatively small, with the ex- 

 posed faces rounded, and largely concealed ventrally by the processes 

 which arise from it. 



Measurements of the periotic hone. 



mm. 

 Greatest length of periotic (tip of anterior process to broken posterior 



margin) 33. 5 



Greatest depth of labyrinthic region of the periotic 11. 5 



Greatest breadth of labyrinthio region of the periotic 19.5 



TYMPAXIC. 



Neither one of these two tympanies is entire. The thin brittle 

 outer lip which arches over the involucrum and the slender processes 

 which project from it are frequently damaged when the tympanic is 

 broken away from the periotic, even in case of the living purpoises. 

 Since the processes which join the tympanic to the periotic are very 

 slender, one may expect to find many broken and otherwise imperfect 

 tympanic bones. 



The left tympanic is badly broken, and some of the missing pieces 

 were not found in the matrix. The fragments which were found have 

 been fitted together (pi. 7, fig. 2), but no restoration has been at- 

 tempted. Comparisons were made with the tympanic bones of some 

 20 genera of living dolphins and only 1 genus, Platanista (pi. 

 7, fig. 1), exhibited any marked agreement. The tympanic bones of 

 this fossil porpoise and Platanista are very similar in general fea- 

 tures, even to the matter of size. Ajnong the other living dolphins 

 available for comparison, Inia showed the closest approach to this 

 type of tympanic. It is unfortunate that the type skull of Lipotes 

 vexillifer lacks both tympanic and periotic bones. 



