1869.| THk MALLEUS AND THE INCUS OF THE MAMMALIA. 401 
is sufficient to prove that the inner extremity of 7 must needs lie 
beneath and internal to the eye, and cannot by any possibility come 
near the fenestra ovalis. It therefore seems to be impossible that 
can be the stapes. i 
Bearing clearly in mind the demonstration now given that the 
stapedial apparatus (if I may so term the stapes with its appendages) 
of the Sauropsida is connected entirely with the hyoidean arch, and 
that it consists of a stem terminating, at one end, in the plate which 
covers the fenestra ovalis, and, at the other, in sundry processes of 
cartilaginous or fibrous texture, one of which is connected with the 
tympanic membrane (when that structure exists), while another 
passes up to be united with the otic region of the skull, close to the 
articulation of the quadrate bone, we may pass to the consideration 
of the homologies of these parts in the ordinary Mammalia, of which 
Man may be taken as an example. 
The Okenian view, adopted by Prof. Peters, assumes that the ramus 
of the mandible of the Mammal answers to the whole ramus of the 
mandible of a Sauropsidan, that the tympanic bone of the Mammal 
answers to the quadrate bone of the Sauropsidan, and that the 
ossicula auditis of the Mammal, or the mal/eus, incus, and stapes, col- 
lectively, correspond with the stapedial apparatus of the Sauropsidan. 
The Reichertian view, which I have hitherto supported, assumes 
that the ramus of the mandible of the Mammal answers only to 
part of the ramus of the Sauropsidan, inasmuch as the arti- 
cular piece of the Sauropsidan mandible answers to the malleus of 
the Mammal—that the quadrate bone of the Sauropsidan is the 
homologue of the ixews of the Mammal—and, consequently, that 
the stapedial apparatus of the Sauropsidan is entirely represented by 
the séapes of the mammal. In the place of the tympanic bone of 
the mammal there are only the ossifications which are found in 
the membranous frame of the tympanic membrane in some Saurop- 
sida (e.g. many birds) and Amphibia. 
The arguments by which this view has been supported are briefly 
these :— 
In the Sauropsidan embryo a rod of cartilage occupies the first 
visceral arch on each side, and meets its fellow in the middle line. 
The rod becomes jointed, and the part on the distal side of the joint 
is converted into Meckel’s cartilage, while that on the proximal 
side of the joint is modelled into the rudiment of the quadrate bone, 
which is invariably, in its earliest state, cartilaginous. Soon, how- 
ever, the quadrate cartilage ossifies, and a centre of ossification 
appears in that part of Meckel’s cartilage which articulates with the 
quadratum. This gives rise to the articular element of the mandible. 
All the other constituents of the lower jaw are developed in the 
fibrous tissue which surrounds the rest of Meckel’s cartilage, which 
structure either persists throughout life, or disappears. 
In a mammalian embryo the first visceral arch also contains a rod 
of cartilage, which, there can be no doubt, is the homologue of that 
in the Sauropsidan. The ramus of the mandible is developed in the 
fibrous tissue which surrounds the distal portion of the rod, which 
