758 
PROFESSOR OWEN ON THE MEGATHERIUM. 
is effected by a transverse bony bar prior to the actual confluence of those pro- 
cesses, and pleurapophyses continue to be attached to some of the parapophyses after 
their transmutation into the haemal arches, as e.g. in the Tunny, the Dory and the 
Salmon*. Nevertheless, in the Salamandroid Polypterus and Lepidosteus\ , and in the 
nearly allied Protopterus and Lepidosiren, the true pleurapophyses gradually bend 
down and unite to form the haemal arches in the tail ; whilst in the Amia calva the 
pleurapophyses coalesce to form the inferior spine, continued from the haemal arch, 
formed by the coalesced parapophyses In the Cod-tribe ( Gadidce) the progressively 
reduced pleurapophyses coalesce with the parapophyses to form the haemal arches. 
In Ophidians, as in Fishes, the transition from the trunk to the tail is not interrupted 
by the modification of any of the vertebral segments to form a ‘ pelvis.’ The ribs 
are supported by diapophyses, with which they become confluent in the tail, where 
some of the pleurapophyses are singularly modified, bifurcate or thickened at the 
extremity, but never bent down and united together to form the haemal arch : as 
there are no bony haemapophyses completing the haemal arch in the trunk, the suc- 
ceeding segments of the tail retain, or have been influenced by, the same modifica- 
tion of vertebral development, and the analogue of the haemal arch is formed, as I have 
before observed, by a pair of parallel exogenous hypapophyses (Plate LII., fig. 28 , hy), 
and is open below, like the incomplete haemal arches throughout the trunk. In 
Lizards and Crocodiles a new vertebral element appears to be introduced to com- 
plete the haemal arch in the chest ; it is, however, only a repetition of an element 
which, under a more developed and modified form, completes the haemal arches in 
the head, in both fishes and reptiles, viz. the ‘ haemapophysis this element is called 
‘ upper’ and ‘ lower jaw’ and ‘ horn of the hyoid’ in the head, ‘ sternal rib’ or ‘ cartilage 
of the rib ’ in the chest : the same element is retained to form the ‘pelvis,’ where it is 
called ‘ pubis’ and ‘ ischium ’ ; and it is continued as a distinct pair of bones in the 
beginning of the tail in the Great Ant-eater (Plate LIIL, fig. 60 , hh), and throughout 
a great part of the tail in the Ichthyosaurus and Plesiosaurus-, whilst the pair of 
haemapophyses become confluent at their distal ends, and form the so-called chevron- 
bones in the Crocodile, and most other Saurians. Under all these modifications the 
haemapophyses coexist with pleurapophyses in the thorax, the pelvis and the tail ; 
only in the latter region the pleurapophyses become anchylosed to the diapophyses 
or to the centrum, and stand out as ‘transverse processes’ (fig. 60 , if, jo/). The 
sudden diminution in the size of the haemal arch in the tail, as contrasted with its 
expanse in the pelvis, is apt at first to excite some doubt as to the serial homology of 
the ‘ chevron bones’ with the ‘ ischia’ and ‘ pubes ’; but when we compare them in a 
species, as e.g. the Mole (Talpa), in which the haemal arch in the pelvis is limited to 
* Cuv. 1. c. p. 224. 
t Muller, Ueber den Ban und die Grenze der Ganoiden, und iiber das naturliche System der Fiscbe, 
1844. 
J “ Omnes arcus inferiores spinam gernnt e costis concretis formatam.” — Franke, ‘ Nonnulla ad Amiam 
Calvam accuratius cognoscendam.’ 
