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ON THE VERTEBRATE SKELETON. 259 
tunny, and salmon yield this striking refutation of the idea of the formation 
of those arches in all fishes, by displaced, curtailed and approximated ribs. In 
some fishes, however (e. g. the cod), reduced pleurapophyses coalesce with the 
parapophyses to form the heemal arches of the caudal vertebra. The meno- 
pome, amongst the lowest or perennibranchiate reptiles, yields a clear disproof 
of the formation of the hemal arch in the tail by the pleurapophyses (the 
rts, viz. called by Geoffroy ‘ paraux, and by Dr. Grant ‘ catavertebral ele- 
ments ’ in the abdomen of fishes)*. The vertebral ribs or pleurapophyses in 
the menopome (fig. 27, p/) are short and simple and suspended to the extre- 
mities of the diapophyses (d) at the beginning of the tail, where they coexist 
with hzemal arches (A, h): these must be formed, therefore, by different ele~ 
ments, which, since no trace of parapophyses exists in any part of the spine, 
I conclude to be the ‘hemapophyses.’ The young crocodile and the adult 
enaliosaurs give the same evidence of the nature of the hzmal arches in the 
tail, with which the corresponding arches or chevron-bones, in cetacea and 
many other mammalia, are homologous. 
_ Thus the contracted hemal arch in the caudal region of the body may be 
formed by different elements of the typical vertebra: e. g. by the parapophyses 
(fishes generally) ; by the pleurapophyses (lepidosiren) ; by both parapophy- 
ses and pleurapophyses ( Sudis, Lepidosteus), and by heemapophyses, shortened. 
and directly articulated with the centrums (reptiles and mammals)+. The 
caudal vertebree of some flat-fishes (Pleuronectide, fig. 16), and the mu- 
reenz, would seem to disprove the parapophysial homology of the hemal arches 
in such fishes, since transverse processes. from the sides of the body coexist 
with them, as they do in the cetacea. But, if we trace the vertebral modifi- 
cations throughout the entire column in any of these fishes, we shall find that 
the hzmal arches are actually parts of the transverse processes ; not independ- 
ent elements, as in the cetacea ; but due to a progressive bifurcation : this, in 
Murena Helena, for example, begins at the end of the transverse processes 
of about the twenty-fifth vertebra, the forks diverging as the fissure deepens, 
until, at about the seventy-third, the lower fork descends at a right angle to 
the upper one (which remains to represent the transverse process), and, 
meeting its fellow, forms the hzmal arch, and supports the antero-posteriorly 
expanded hzmal spine. In the plaice a small process is given off from the 
expanded base of the descending parapophysis of the first caudal vertebra, 
which increases in length in the second, rises upon the side of the body in 
the third, becomes distinct from the parapophysis in the fourth, and gradually 
diminishes to the ninth or tenth caudal vertebra, when it disappears. These 
spurious transverse processes never support ribs. 
' The neurapophyses are often directly perforated by the nerves in fishes, 
but are sometimes notched by them, or the nerves issue at their interspaces. 
_ The neurapophyses, which do not advance beyond the cartilaginous stage in 
the sturgeon, consist in that fish of two distinct pieces of cartilage; and the an- 
terior pleurapophyses also consist of two or more cartilages, set end on end: and 
this interesting compound condition is repeated in cases where the pleurapo- 
physial element is ossified and required to perform unusual functions in the 
bony state in other fishes. Amongst the more special or exceptional modifi- 
cations of the vertebre of the trunk of fishes, which indicate the extent to 
which their normal segmental character may be marked, I would cite those 
-of the anterior vertebre in the pipe-fishes, in the loaches, and in certain 
siluroids. 
* Outlines of Comparative Anatomy, p. 58, fig. 28, B, g. 
+ By a misconception of the sense in which I use the term ‘ hemapophyses,’ M. Agassiz 
has applied it to the lamin of the inferior or hemal arches in fishes. ‘‘ Recherches sur les 
Poiss. Foss.” tom. i. p. 95. 
