158 PROFESSOR OWEN ON THE DEVELOPMENT AND HOMOLOGIES OF THE 
that a distinct piece h! is interposed between the pleurapophysis {pi) and hseina- 
pophysis (A), and it is less completely ossified than either of those elements. The 
sternum hs is a single symmetrical rhomboidal plate, of which a narrow median por- 
tion only is completely ossified. With the endo-skeletal segment is combined, in 
the figure, parts of the corresponding ossified segment of the exo-skeleton, which 
parts are covered, like the expanded parts of the carapace of the Chelonia, by 
thick cuticular scutes. According to this analogy, c being the centrum and ns the 
neural arch and spine, sc answers to the detached dermal bony plate sc in fig. 4. 
The head, neck and continuous slender part of the rib {pi, fig. 6) answers to the 
pleurapophysis {pi) in fig. 4, and the expanded plate {sc', fig. 4) answers to the lateral 
bony dermal plates {sc', sc', fig. 6): the marginal plate A, A', fig. 4, occupies the place of 
the intercalated costal piece A', fig. 6 : the hyosternal A, ^s, fig. 4, answers to the heem- 
apophysis or ossified cartilage of the rib (A, fig. 6), the other parial pieces also being 
expanded hsemapophyses ; and the entosternal lis (fig. 4) alone represents the simple 
sternum /is in the Crocodile: in brief, the figures within the segment fig. 4, indicate 
the homologies according to the Crocodile (fig. 6), those without or below the segment 
(fig. 4) indicate the homologies according to the Bird, fig. 5. 
In this comparison it will be seen that the mesial end of the costal plate (^c', fig. 4), 
which quits the rib to articulate with the vertebral plate (^c) in the Tortoise, is not the 
homologue of the tubercle of the rib which articulates with the diapophysis c?, fig. 6, 
in the Crocodile: the true endo-skeletal pleurapophyses, or vertebral ribs, of the 
Chelonians I regard as being simple, and articulated by a head only to the central 
part of the vertebra, as in other Reptilia which have but one ventricle of the heart. 
They are almost straight, and so far resemble the free ribs (pleurapophyses), which 
project from a few of the dorsal vertebrae in the Pipa or Surinam Toad. 
Were the large and complex abdominal haemapophyses of the Plesiosaur (fig. 7, A) 
to coalesce on each side, they would form two lateral masses with their extremities 
projecting outwards and inwards, like the teeth of the hyosternals {As) and hypo- 
sternals {ps) in the plastron of the Turtles and Trionyces (figg. 3 and 8). 
In olfering the comparison of the thoracic-abdominal segment of the Crocodile 
with that of the Chelonian to the consideration of Comparative Anatomists, my ob- 
ject has been rather to show that the subject admits of more than one view, and re- 
quires further investigation, than to substitute merely by such comparison a different 
homological hypothesis from that whieh has hitherto prevailed in this country ; being 
conscious that without the illustrations of which such hypothesis may be susceptible, 
it would be of as little real avail in attaining to a true knowledge of the vertebrate 
organization of the CJielonia as the similarly unconfirmed view of Geofproy St. Hi- 
laire must be considered to be. The guide to our choice of either of these, or of any 
other view that has been offered of the nature and signification of the thoracic- 
abdominal case of the Chelonia, must be the light afforded by a true perception and 
explanation of the phenomena of its development. 
