290 



Dr. A. A. Gray. 



[Jan. 26, 



respectively at right angles, because there is none of that curving downwards 

 to meet each other as they approach, such as is found in all other mammals 

 which have been examined. Similarly, the canals as they approach their 

 ampullary extremities do not curve towards the vestibule, but turn suddenly 

 downwards at right angles a short distance before they dilate into the 

 ampullae. 



The sloth, in common with man, the monkeys, and the seal, has a well- 

 developed perilymphatic space in all the semicircular canals. According to 

 the view expressed by the writer this indicates a relatively ancient type of 

 labyrinth. 



The internal diameter of the superior canal measures 3 mm., and the 

 external diameter 4-5 mm. The height of the vertex of the canal above the 

 vestibule is 2 - 25 mm., and the diameter of the canal itself at the vertex is 

 1 mm. The internal diameter of the posterior canal is 2 - 5 mm., and the 

 external diameter is 4*5 mm. The height of the vertex of the canal from 

 the vestibule is 2 - 5 mm., and the diameter of the canal itself at the 

 vertex is 1 mm. The external canal is much the smallest of the three, 

 measuring only 1/5 mm. in diameter internally and 3 - 5 mm. externally. 

 The height of the vertex of this canal above the vestibule is only 1*5 mm., 

 and the diameter of the canal itself at the vertex is 1 mm. 



The angles at which the canals diverge from one another are smaller than 

 in any of the mammals which have been examined and this gives the canals 

 the appearance of having been pressed somewhat together. In addition to 

 this it will be seen, on comparing the measurements or on examining the 

 photograph, that the vestibule and canals occupy a smaller proportion of the 

 whole labyrinth than in most mammals. It is exactly the reverse, for 

 example, of the condition found in the lemur, where the canals are very 

 long and slender. It may be that this small size of the canals, associated 

 with their irregular shape, may be in some way related to the sloth's clumsy 

 and slow movements. The life which they lead, with the body inverted, as 

 it almost continually is, may also be connected in some way with the curious 

 development of these organs. 



The Labyrinth of the Brush-tailed Wallaby (Petrogale penicillata). 

 (Plate 17, fig. 4.) 



The labyrinth of the marsupials is not so divergent in structure from that 

 of the general type of mammalian labyrinth as might be expected in an 

 order of animals so far removed from most of the present divisions. It is, for 

 example, less peculiar than that of the sloth and far less peculiar than that of 

 the monotremata, the cetacea, or the seal. 



