PROFESSOR OWEN ON THE MEGATHERIUM. 
757 
as the homologue of that arch. The perforated haemal arch in the cervical vertebrae 
of the Pelican, and the parallel descending- processes similarly protecting the caudal 
artery in the Python (Plate LI., fig. 28 ), are alike exogenous growths from the cen- 
trum, and are serially homologous with the hypapophysis, not with the haemapophysis. 
The total absence of true ossified haemapophyses throughout the trunk and tail 
vertebrae, is a striking characteristic of the Ophidian reptiles % as it is of fishes. 
The determination of the homology of the autogenous haemal arches in the tail 
carries with it that of the question of their relation to the inferior and sometimes 
perforated exogenous processes of the centrum in more advanced parts of the verte- 
bral column. 
The vast difference in point of size between the so-called ‘ chevron-bone ’ of the 
Crocodile and the bony arch formed by the sternal and vertebral ribs of a thoracic 
vertebra, seems at first sight to discountenance the idea of any homology between 
them. But the difference is not greater than that between the neural arch of the 
parietal segment of the skull in Man, and the arch formed above the middle dorsal 
vertebra : yet I think it may be now assumed as a settled point in anatomy, that 
those arches are homotypes or serial homologues. The difference in point of size is, 
indeed, much less in the cold-blooded and small-brained Vertebrates than in Man ; 
but the transition from the abdominal to the caudal hseraal arches in fishes is much 
more gradual than that of the cranial to the abdominal neural arches in the same 
class, whilst the similarity of the neural and haemal arches to one another is so close 
throughout the region of the tail, as to impress the mind with a conviction that they 
are alike the result of modified annular elements, in short, ‘vertical homotypes.’ 
Cuvier, accordingly, designates them by the same descriptive phrases-f-. 
The general proposition that the ‘chevron bones’ or ‘inferior annular part and 
spine’ of the vertebrae in the tail of fishes, are modifications of the haemal arches in 
the trunk, may be deemed to be unassailable. But the haemal arch is a complex 
whole, and may include diapophyses or parapophyses with pleurapophyses, haemapo- 
physes and haemal spine. In the abdomen of fishes the bony arch is incomplete 
below, and in its most complex state consists only of parapophyses and pleurapo- 
physes. With the exception of the clavicular and pubic arches, bony haemapophyses 
are developed in no part of the trunk of fishes. When, therefore, we rigidly scru- 
tinize the composition of the haemal arches in the tail, we find them composed of 
parts quite distinct from true haemapophyses, and also composed of different 
vertebral elements in different species of fishes, a fact which adds to the proof 
of the essential serial homology of those arches in the tail with the haemal arches 
in the trunk. In many osseous fishes the haemal arches in the tail are formed by the 
gradual bending down and coalescence of the parapophyses ; in some the union 
* Cartilaginous appendages to the pleurapophyses in Serpents maybe viewed as rudimental ‘ hsemapophyses.’ 
t “ Les caudales ont une partie annulaire et une apophyse epineuse en dessus et en dessous.” Cuviek, 
' Le9ons d’Anat. Comp.’ Ed. 1835, t. i. p. 223. 
MDCCCLI. 5 E 
