582 
Proceedings of the Royal Society of Edinburgh. [Sess. 
XXXV. — The Development of the Auditory Ossicles in the Horse, 
with a Note on their possible Homologues in the Lower 
Vertebrata. By Ray F. Coyle, B.S. (From the Zoological 
Department of the University of Edinburgh.) Communicated 
by Professor J. C. Ewart, M.D., F.R.S. (With Six Plates.) 
(MS. received May 25, 1909. Read July 5, 1909.) 
I. 
The discussion as to the homologies existing between the sound-conduct- 
ing apparatus of the mammalia and certain elements of the lower jaw and 
branchial skeleton of fishes has occupied the attention of numerous authors. 
In fact the question may be considered as a classic, dating as it does to at 
least 1778, when Geoffry published his Dissertations sur Vorgane de Vouie 
de I’homme, des reptiles, et des poissons. From that time until 1898 — from 
Geoffry to Gaupp — the contributions to the literature upon this subject 
constituted an enormous amount. Inasmuch as Gaupp has given a detailed 
and masterly review of the literature up to the time of his work (1898), and 
since Fuchs thoroughly covered the ground seven years later, I shall not 
undertake that task in the present communication. Moreover, as this paper 
is in the nature of a preliminary report upon the entire developmental 
history of the skull in the horse, I shall not here review in detail the con- 
tributions to the literature since the time of Fuchs’ account (1905). 
A specialised apparatus for the conduction of sound to the auditory 
organ first makes its appearance as the stapedial plate (operculum) and 
columella auris of urodele amphibians. Amongst many Anura the spira- 
cular cleft, which disappears entirely during the development of the Urodela 
and Gymnophiona, becomes “ modified to form a Eustachian tube, opening 
into the pharynx and leading into a tympanic cavity. The latter is closed 
externally by a tympanic membrane supported by a cartilaginous ring 
(annulus tympanicus), and to it the distal end of the columella is attached ” 
(Wiedersheim). In the Mammalia the sound-conducting apparatus consists 
of an osseous malleus, articulated with the tooth-like incus. The incus 
articulates by its long arm (crus longum incudis) with the stapes (fig. 5). 
Many investigators have assigned the stapedial plate and accessory 
cartilages of the amphibia to the second visceral arch, making it thus 
possibly homologous to the hyomandibula of the fishes. Fuchs, however, 
has clearly shown that the stapedial plate is developed from the auditory 
