260 MR. ST. GEORGE MIVART ON THE [Apr. 28, 
7. On the Axial Skeleton of the Urodela. 
By Sr. Grorce Mrvart, F.R.S. 
In this communication I shall confine my observations to the spinal 
column, neglecting for the present the skull, together with the man- 
dibular, hyoidean, and branchial arches*. 
* In a communication read before the Linnean Society on the 21st of April 
of the present year, I stated my views as to the general and serial homologies of 
the vertebrate skeleton. 
I said that the axial system in its most developed form might be considered 
as consisting of three longitudinal series of parts, continued for a greater or less 
extent along each side of the vertebral axis. 
The upper longitudinal series of parts on each side together form the system 
of epaxial parts. 
The middle longitudinal series of parts on each side together form the system 
of paraxial parts. 
The inferior longitudinal series of parts on each side together form the system 
of hypaxial parts. 
Epaxial parts were defined as “parts embracing the cerebro-spinal axis, or 
parts serially homologous with parts which embrace that axis.” 
Paraxial parts were defined as ‘ parts external to and more or Jess tending to 
embrace the pleuro-peritoneal cavity, or parts serially homologous with parts 
which are so situated in the trunk.” 
Hypaxial parts were defined as ‘‘ parts placed between the skeletal axis and 
some part of the pleuro-peritoneal cavity, or serially homologous with parts so 
situated. 
By epaxial parts I denote the neural arches and lateral walls of the brain-case. 
By paraxial parts I denote transverse processes, ribs (both upper and lower 
of fishes), and sternal bones or cartilages. 
By hypaxial parts I denote both hypapophyses, whether exogenous or auto- 
genous, and also visceral arches, however complex. 
By ‘visceral arches” I mean the system of arches forming the hyobranchial 
apparatus, and also the mandible, the palato-quadrate arch, and the trabecule 
cranii. 
In this latter respect I follow the happy and original suggestion of Professor 
Huxley, as far as regards the resemblance of the trabecule cranii to the other 
visceral arches; but I venture to differ from him as far as regards the general 
homology of these visceral arches, which I regard not as ribs (¢. e. paraxial parts), 
but as hypaxial elements of the skeleton. The position of the heart and aortic 
roots, with regard to the branchial arches, is, I submit, fatal to their costal 
character. 
The external branchial cartilages of Sharks, and the branchial basket of the 
Lamprey, however, may really represent costal elements, 
In the paper referred to, I gave my reasons for considering the subcaudal 
arches of fishes to be generally of more or less mixed paraxial and hypaxial 
nature, differentiation, in this respect, being, I believe, less complete in that 
class. 
This radiating system of skeletal parts of the trunk corresponds to the radiating 
system of spinal nerves, first pointed out by Professor Huxley in his last course 
of Hunterian Lectures, the epaxial parts corresponding with the ascending 
nerves, the hypaxial parts with the nerves of the inner ventral laminz of the 
embryo (i. ¢. with the sympathetic), and the paraxial parts corresponding with 
the nerves of the outer ventral laminze. Moreover, as there are nerves passing 
directly outwards (above the abdominal nerves), so it was suggested that there 
may be an upper and lower series of paraxial parts, perhaps coalescing to form 
the ribs of the higher vertebrata. 
If this view of the visceral arches be accepted, then the nerves accompanying 
