PROFESSOR OWEN ON THE MEGATHERIUM. 
747 
that those between the normal articular processes, or zygapophyses, are suppressed. 
The true serial homology of the processes {p. p.) as ‘ parapophyses,’ developed from 
the fore-part of the base of the neural arch to articulate with the under part of the 
anapophyses, is well illustrated by the vertebrae of the Great Ant-eaier, and, as will 
be afterwards shown, in the Megatherium, in which the true diapophyses are better 
developed than in the Armadillos. 
Definite terms for those several processes that undergo such remarkable modifica- 
tions in the great Edentata become indispensable ; they could scarcely be rendered 
intelligible by the ordinary descriptive periphrases. But, by means of single sub- 
stantive names, all their modifications can be defined, and, by adjectival inflections 
of those names, the articular surfaces can be distinguished. 
In figures 21, 22 and 23, Plate L., initial letters indicate the processes on one side, 
and the articular surfaces on the other side of each vertebra, thus : — 
m is the metapophysis, with m a its ‘ anapophysial,’ and mz its zygapophysial ’ 
articulations. 
d is the diapophysis, w th dpi its ‘ pleurapophysial,’ and da its ^ anapophysial’ 
articulations. 
p is the parapophysis, with p a its ‘ anapophysial’ articulation. 
a is the anapophysis, with am its ‘ metapophysial,’ ap its ^ parapophysial,’ and 
a c? its ‘ diapophysial’ articulations. 
2 and z', in figs. 21 and 22, indicate the ordinary anterior and posterior zygapo- 
physes ; they become very small in the middle dorsal vertebrae, and are sup- 
pressed in the posterior dorsals and lurnbars ; the process z' in figs. 22 and 
23 is not a gradual modification of the posterior zygapophyses, a' in fig. 21, 
but is a substitution for them, called out, as it were, by the new surface mz 
upon the metapophysis. 
As the complex structure of the vertebrae of the Armadillos and Ant-eaters has, 
hitherto, been sought to be illustrated by reference to the vertebrae of Serpents, it 
becomes necessary, after the foregoing analysis, to ascertain how far the analogy 
holds good. 
The Ophidian reptiles so far resemble the Mammalia in their vertebral characters, 
that all the autogenous elements, except the pleurapophyses (Plate LI., fig. 31, 32, 
pi), coalesce with one another in the trunk, and the pleurapophyses {lb., fig. 28, pi) 
themselves become anchylosed to the diapophyses {ih. d) in the tail. But the 
exogenous processes present many differences from those in Mammalia. The dia- 
pophysis (figs. 24 and 25, d) extends from the fore-part of the side of the centzuim, 
and is large, vertically oblong or hemispherical (fig. 38, d), and very short : it is 
covered by the articulation for the head of the rib*. The base of the neural arch 
* This may be the ‘ head ’ of the rib in the same sense in which it would be said of that end of the simple rib 
which articulates with the diapophysis in the hinder dorsal vertebrae of the Crocodile. Guided by this modih- 
cation of the ribs in that reptile, the Anatomist would be led to the homology of the simple ribs, and of the 
