1906.] Observations on the Labyrinth of Certain Animals. 



295 



diameter of the tube of the cochlea just in front of the round window is 

 rather more than 2 mm. 



The vestibule is an irregular cavity and measures a little more than 3 mm. 

 in its greatest diameter. It contains two large otoliths. Both of these are 

 flat. The largest, which lies close to the ampullary openings of the superior 

 and horizontal canals, is roughly circular in shape. The second and smaller 

 one is almost square and lies about 1 mm. below and internal to the first. 

 Both the otoliths are milk-white in colour. The oval window measures 

 2 - 5 mm. in its longest diameter. 



As far as can be judged from the broken specimen, the superior canal is in 

 the form of an ellipse, with the longest diameter lying backwards and 

 upwards. It is much the largest of the three, measuring 7'5 mm. in' its 

 internal and 11 mm. in its external diameter. The height of the vertex of 

 the canal above the vestibule is about 9 - 5 mm. and the diameter of the canal 

 itself at the vertex is l - 5 mm. The posterior canal is also in the shape of 

 an ellipse, with the long diameter in a vertical plane. It measures 5 - 5 mm. 

 in its internal and 8'5 mm. in its external diameter. The height of the 

 vertex of the canal above the vestibule is only 3"5 mm. and this gives to 

 the canal a somewhat squat appearance. The diameter of the canal itself at 

 the vertex is 1*5 mm. The horizontal canal is the smallest of the three and, 

 like the posterior, is of a squat appearance. It measures rather more than 

 4 mm. in its internal and 8 - 5 mm. in external diameter. The diameter of the 

 canal itself at the vertex is 1*25 mm. 



A very interesting feature of the canals of the ostrich is the fact that 

 there is no communication between the posterior and horizontal canals at the 

 point at which the arch of the latter passes under that of the former. 

 There is a distinct though narrow interval between them which is filled up 

 with bone in the unprepared subject. This feature of the labyrinth of the 

 ostrich is unique in birds so far as present investigations have shown, but it 

 may be found in other birds of the same class. A specimen of the rhea is 

 now in course of preparation and it will be interesting to see if this feature 

 is repeated in that bird. 



EXPLANATION OF PLATES 16—18. 



Each labyrinth is represented by two plates. The upper plate is a half-tone repro- 

 duction from a photograph of the organ. The finer details are shown in this plate, but 

 the three dimensions which the organ occupies are not of course appreciable and the 

 comparative sizes are not represented, as the degree of magnification varies in each. The 

 rough outline drawings below give, to a certain extent, the sense of three dimensions. 

 The comparative sizes of the objects are also shown in these drawings, each being 



