370 MR. ST. GEORGE MIVART ON THE VERTEBRATE SKELETON. 
8. According to relations of bilateral symmetry—as when we speak of “ the right 
clavicle.” 
9. According to relations of vertical symmetry—as when we speak of “ the upper and 
lower arches of the tail of a flat fish,” e. g. of a sole. 
10. According to radial symmetry—as when we speak of “the processes diverging in 
various directions from a single vertebral centrum.” 
11. Generally, or according to the place or rank occupied by a part, or series of parts, 
in an abstract ideal vertebrate skeleton—as when we speak of **a centrum," or “a neural 
arch." Such an abstract ideal, to be a suitable typical conception, should be the “ simplest 
possible generalized expression of observed facts’’*. 
Now it is to a certain extent of special, but more of serial and general relations that 
I have to speak; and these have been so well defined and described by Professor Owen t 
‘under the happily chosen terms special homology, serial homology, and general homo- 
logy, that little more, I think, need here be said with regard to them. 
In investigating the special homology of skeletal parts, the test employed must be the 
connexions or contiguity existing, at first or ultimately, between the parts in question 
and other struetures in the two or more animals compared. 
In investigating serial homology the test employed must be the resemblance at first or 
ultimately existing between a certain skeletal part of some animal and other parts, in the 
same animal, in front of or behind that certain part. No other test than that of parts 
being or not being “ in series” is, I believe, possible in questions of serial homology. 
In investigating general homology, both the foregoing tests must be employed, and 
applied to the several parts of all the more divergent vertebrate animal forms. 
The endo-skeleton is, at least conveniently, divisible into, 1, the axial skeleton, and, 2, 
the appendicular skeleton. Such a division I believe to be not only convenient but also 
natural. 
THE AXIAL SKELETON. 
The parts which together make up the axial skeleton are not yet all satisfactorily 
grouped and classified, as regards their general homology; and this remark especially - 
applies to some of the inferior arches. Before, however, the general homology of these 
arches can be investigated with any chance of success, it is necessary to determine rela- 
tions of special and serial homology. 
The uncertainties which hang over these inferior arches have been forcibly brought 
home to my mind in studying the skeleton as it exists in tailed Batrachians—animals 
presenting such interesting resemblances to Fishes on the one hand, and to Reptiles and 
Mammals on the other. 
To endeavour, if possible, to contribute towards the removal of some of these uncer- 
tainties, I have been led to review the labours of previous writers, and to reexamine and 
reconsider observations of my own. The results I venture now to present, in the hope 
* Prof. wu Croonian Lecture, 1858. 
7 t Bruns e and Homologies of the yati ibiti. pp. 1 & 72. 
