162 PROFESSOR OWEN ON THE DEVELOPMENT AND HOMOLOGIES OF THE 
In the embryo Testudo indica the uniformly slender pleurapophyses {d 1 — d9, fig. 5) 
are ossified nearly throughout their whole length before the ossification of the costal 
plateSj usually regarded as their expanded tubercles, commences : and the beginning 
of the superadded bone* is not at the same point in each rib, as might have been ex- 
pected if it were the exogenous process called ‘ tubercle ’ of the rib. The costal 
plates are situated in the young Testudo indica (Plate XIII. figs. 4 and 5, ph — s) 
alternately nearer to and farther from the head of the rib ; and their presence seems 
to be determined rather by the angle of union of the superincumbent vertebral 
scutella with the lateral or costal scutella, than by the necessity for additional 
strength in the articulation of the ribs with the spine. Ossification commences at the 
point from which the three impressions radiate, and as this point is nearer the median 
line at the median apex of the costal scutellum than at the lateral apex of the verte- 
bral scutellum, the resulting plates of bone are alternately further from or nearer 
to the middle line ; and the first, third and fifth costal plates have advanced 
along the proximal end of the rib so as to join the neural plates, whilst the second, 
fourth and sixth costal plates leave a portion of the proximal end of the rib uncovered 
and crossing the space between the incipient costal plate and the neural plate./ In 
regarding these incipient ossifications, extending into the substance of the corium 
and receiving the impressions of the epidermal scutes as the developed ^tubercle’ of 
the ribs, as Rathke has endeavoured to illustrate in Tab. III. figs. 11 (Tortoise), 12 
and 13 (Chick) of his elaborate Monograph f, we are compelled to suppose that each 
successive rib in the Tortoise has a different position of its tubercle, which is alter- 
nately nearer and farther from the head, and that the neck of each successive rib is 
alternately long and short, which is contrary to all analogy furnished by those cold- 
blooded or warm-blooded Vertehrata that have unquestionably the exogenous pro- 
cess called ^tubercle’ developed from the true neck of the rib. 
When the partially ossified carapace of a young tortoise is dried, one cannot fail 
to be struck with the difference in the texture and external suj-face of the bones which 
unquestionably belong to the endo-skeletal vertebrae, and of those which, notwith- 
standing their connection with the neural spines and pleurapophyses, are developed 
in the fibrous substance of the corium. These nascent ‘neural’ and ‘costal plates’ 
of the carapace have a granular exterior and a coarse spongy texture, whilst the 
neural arches and pleurapophyses are compact, smooth, and with a polished external 
surface : the part of the pleurapophysis (PI. XIII. fig. 5, d2 — d9) which passes beneath 
and is attached to the under surface of the ‘costal’ plate {pi i — pi 8), contrasts 
strongly with that superimposed dermal ossification. 
The marginal plates {m 1 — m 1 l)present the same rough, coarse, granular character 
as the neural and costal plates : they are in no way connected in their development 
* This period, in its relation to the development of the neural arches and pleurapophyses, corresponds pre- 
cisely with that at which the dermal plates of the Crocodile begin to be ossified. 
t Ueber die Entwickelung der Schildkrdten, 4to, 
