520 Dr. A. A. Gray. On the Anatomical [June 1, 
to be deposits like otoliths immediately internal to the oval window, but the 
fact that these are otoliths cannot be definitely stated. For reasons above 
given the ordinary examination of the organ by microscopic sections cannot 
throw any light upon the matter. 
In one respect, however, the vestibule of the echidna’s labyrinth is 
distinctly more like that of mammals than that of at least the majority of 
reptiles. Its size, relative to the rest of the organ, is small, whereas in 
reptiles the vestibule is frequently the bulkiest portion of the whole 
labyrinth and occupies almost all the space contained by the planes of the 
three canals. 
The semicircular canals of the echidna are very definitely mammalian in 
type (fig. 10, s.p.). They have entirely lost that angularity which is so 
characteristic of those of the reptile and of which vestiges are found in the 
platypus and the sloth.t Further, the junction of the superior and posterior 
canals is exactly similar to that found in mammals, with the exception of the 
platypus and sloth. In these two animals, as also in the reptiles, the two 
canals meet one another abruptly to form the common crus, whereas in the 
echidna and in mammals generally they approach one another with a gradual 
curve, so that when they unite a forked disposition of the union is produced. 
The perilymph space in the canals is fairly well marked in the echidna, 
a feature which is invariable in the reptiles hitherto examined, but rather the 
exception in the mammals. The evolutionary significance of the perilymph 
space in the canals has already been discussed.j It was suggested that the 
original condition in mammalia presented the appearance of a perilymph 
space of considerable dimensions, as large, for example, as the endolymph 
space. In the very great majority of mammals, however, this space in the 
canals has been very nearly obliterated, but is still present in the primates, 
including man, in some of the marine carnivora, and in the edentates. Its 
existence in the echidna lends support to the view that originally the 
mammalian stock retained this space, inherited from their reptilian ancestor. 
The space is always present in birds, so far as present investigations have 
shown. In all animals which possess the space, it is almost entirely 
confined to the concave portion of the canal. 
The white deposits seen in the canals in fig. 10 may be calcareous in 
character, but they can hardly be considered true otoliths, since, placed as 
they are at the vertices of the canals, they could have no physiological 
function similar to true otoliths; for there is no nerve supply to any portion 
of the canals except the ampulle. The deposits seen in fig. 10 may be the 
* Gray, op. cit., vol. 2, p. 81, and vol. 1, p. 159. 
+ (Grayop: cits volun. 24. 
