MR. ST. GEORGE MIVART ON THE VERTEBRATE SKELETON. 375 
LIII. fig. 9, é, c). There are also in many fishes additional bony rays (as notably in the 
Herring), generally admitted to be of quite subordinate importance, and which, as far as 
I know, are not represented, at least by hard structures, in any higher class. 
In all the higher Vertebrata the ribs, and in fishes the lower series of ribs, lie, in the 
trunk, in close proximity to the outer wall of the pleuro-peritoneal cavity. "These then 
are hard parts developed in the layer of a membrane enclosing that cavity. Another 
layer of membrane extends from the skeletal axis outwards, above the former; and in 
this layer (in fishes, at all events, if not also in higher vertebrates) hard parts are also 
sometimes developed. 
It occurs forcibly to the mind, then, that the paraxial system of each side may be of 
an essentially duplex nature; and I venture to suggest for inquiry whether the faets of 
comparative anatomy, as regards these parts, may not be well and conveniently grouped 
together by some such conception as the following. 
That there are really two series of paraxial elements on each side of the body :— 
l. An upper series, which, leaving the central axis, tends outwards towards the skin 
(corresponding with the nerves passing directly outwards), and which may be termed 
upper paraxial or (making use of a name employed by Professor Owen) diapophysial 
elements (fig. 2, t, tr, tr’); 
2. A lower series, which, leaving the central axis, tends downwards (corresponding with 
the abdominal nerves) to embrace the visceral cavity, or is serially homologous with 
other parts which do so, and may be called lower paraxial or (again after Professor Owen) 
parapophysial elements (fig. 2, c, cr, vr, sr, & s). And :— 
That these two series of parts frequently coalesce (before or after solidification) for 
à longer or shorter distance, and by so doing give rise to double (superimposed) trans- 
verse processes with ribs bifurcating at each end (Plate LIII. figs. 14 & 15), or to ribs 
with the proximal bifurcation modified into “head” and “tubercle,” and with the 
distal bifurcation disguised by having only one limb of such bifurcation ossified, the 
other remaining membranous, or, finally, to bifurcating transverse processes simply 
(figs. 12 & 13). : : 
According to this conception, the root of each upper paraxial element (when a solid 
structure) is the cartilage or bone (however ossified or wherever situated) connected with 
the tubercle of the rib of higher vertebrates; and it may be termed the upper transverse 
process, or the tubercular process, or the diapophysis * (fig. 2, 4, & fig. 14). ; 
This is continued outwards, in many fishes, by an upper series of ribs (fig. 9, $ fr )— 
in other vertebrates by entering for a certain extent into the composition of the ribs, and 
thence onwards by a bifurcation of the rib, in Amphibiat (figs. 14 € 15), or, in the highest 
vertebrates, possibly by membranes passing upwards from the ribs external to the erector 
spine, or outwards from the ends of the caudal transverse processes. 
The root of each lower paraxial element (when a solid structure) is the cartilage or 
bone (however ossified and wherever situate) which is connected with the head of the 
* Professor Owen originally instituted this term to denote a process essentially exogenous. See * Archetype and 
Homologies,’ p. 82. Le 
t Are the costal processes of Birds and Crocodiles similar? 
