470 BAUR. [Vor. III. 
The question now is, what is the homologue of the interca- 
lare? It is absent in all forms in which the cranium is not or 
only little ossified, the Holocephali, Selachii, Dipnoi, Chondros- 
tei. In Polypterus, the living representative of the Crosso- 
pterygia, the intercalare appears to be a large element, in which 
the paroccipital may be contained. I have not examined this 
form. From the papers of Agassiz, Mueller, and Traquair, I 
could not form an exact opinion, but from notes and drawings 
very kindly sent to me by Dr. E. Koken of Berlin, I have 
reached the conclusion that Po/ypterus is an aberrant form, the 
morphology of which is not yet fully understood. In the Amii- 
dz the intercalare appears for the first time, and from this 
family the “Teleostei” have partially inherited it. Fossil fishes 
do not throw any light upon this question. 
We have seen that in the lowest “ Stapedifera”’ the stapes is 
largest, and that it becomes more and more reduced in the 
higher forms. There can be no doubt that the ichthyic an- 
cestors of the Batrachia must have possessed an element 
corresponding to this well-developed stapes. I think that the 
intercalare represents this element. The stapes is a true bone 
of the cranium; it is not formed by any part of the visceral 
arches in the Batrachia, and all such connections in the higher 
forms must be considered as secondary. It is possible to 
understand now why the opinions on the origin of the stapes 
differ so immensely. 
The “tympanic bone” in Mammals has no homologue in the 
Monocondylia, with the exception of some Birds where a similar 
structure is to be seen (Psittaci, part); it is very small in the 
Monotremata and becomes specialized in the higher groups. 
The opinion of Peters and others, lately defended by Dr. H. 
Gadow, that the tympanic represents the quadrate, cannot, 
I think, be adopted. There seems to me to be very little doubt 
that the quadrate of Mammals is represented by that part of 
the squamosal to which the lower jaw is articulated, since we 
have the same condition in the Theromora, which are nearest 
the ancestors of Mammals. 
I append, in a tabular form, the views of Professor Huxley 
and myself, side by side. 
