-1908.] Structure and Relationships of the Labyrinth. 519 
corresponds to the perilymph recess of the reptile and the bird, and may be 
allowed to retain that name, 7.p. On the outer surface of this perilymph 
recess is the round window, fv., and, as if further to express its reptilian 
character, the round window is markedly oval in shape. The long axis of the 
round window lies in the horizontal plane, as is the case in the reptile and 
the bird, and the membrane which closes the window looks directly into the 
tympanic cavity as in other animals. 
On passing towards the cranial cavity the perilymph recess becomes 
narrowed to form the aqueduct of the cochlea, p.a., which, in its further 
course towards the cranial cavity, lies above the jugular vein. 
The vestibule in the echidna is also similar to that of the reptile except in 
the matter of size. The saccule forms the larger division of the vestibule, 
and in this animal, as in all mammals and reptiles that have been examined, | 
the oval window abuts upon the saccule, the utricle being above and behind 
the latter. In the echidna the oval window, /fo., is almost completely 
circular in shape, in this respect agreeing with that of the bird and many 
reptiles, and differing from that of the mammalia, though in certain of the 
marsupials there is a tendency towards the circular shape. In the echidna, 
therefore, as also in some reptiles and in birds, the oval window is round, and 
the round window is oval. 
There is yet another primitive feature in the labyrinth of the echidna 
which has hitherto escaped recognition. That is the existence of a very 
definite recessus utriculi. In the echidna the recessus utriculi, 7.w., consists 
of a conical diverticulum from the utricle just where the ampulla of the 
horizontal canal opens into the utricle. The blunt and roughly cone-shaped 
recessus utriculi passes horizontally backwards and ends in a nipple-like 
projection just external to the middle of the outer wall of the utricle. 
So far as the writer is aware, the presence of a recessus utriculi has not 
been suspected in any of the mammalia and certainly it has not been found 
in any of the mammals described by him. In the reptile and amphibian it 
does exist, but does not always form a prominence on the outer surface of 
the labyrinth, since the surrounding cavity of the perilymph rounds it off. 
In some birds, however, it forms a prominence in exactly the same position 
as in the echidna, and both in the latter and in the bird its floor is supplied 
by a nerve.* 
The extent to which otoliths are present in the labyrinth of the echidna 
cannot be decided from the specimen in the writer’s possession, for the reason 
above stated, viz., the very long period over which the preparation remained 
in spirit before the organ was extracted. In fig. 10 there certainly appear 
* Op. cit., vol. 2, p. 188. 
