614 REPTILIA. 
from—(a) the flattened neural spines (plus dermal scutes) ; 
(b) expanded and more or less coalesced ribs (plus costal 
dermal scutes); (c) a series of dermal marginal scutes around 
the outer edge. In the Athece the dorsal vertebra and ribs 
are not fused to the dermal plates which form the carapace. 
The ventral shield or plastron ts formed of nine or so dermal 
bones. There ts no sternum. 
Overlapping, but not corresponding to the bony plates, there 
are (except in Trionychia and Athece) epidermic horny plates 
of “tortoise shell,” which, though very hard, are not without 
sensitiveness, numerous nerves ending upon them. 
The quadrate ts immovably untted 
with the skull. There is only a 
lower temporal arcade. The jaws 
are covered by a horny sheath, and 
are without teeth, though hints of 
these have been seen in some em- 
bryos. There is a single anterior 
nasal opening. The scapular arch 
is internal to the ribs. The limbs 
are pentadactyl, but often in the 
form of paddles. 
The average life of Chelonians is 
sluggish. Perhaps this is in part 
due to the way in which the ribs 
are lost in the carapace, for this 
Fic, 331.—Carapace of yyust tend to make respiration less 
tortoise. “active. The lungs are divided into 
The dark contours ae these of a mumber of compartments. 
Citas siesta of theme: . Tae slamcal aperture is usually 
which have been removed. longitudinal, never transverse, the 
copulatory organ is unpaired. 
All are oviparous. The eggs have firm, usually calcareous, 
shells. 
Some Peculiarities in the Skeleton of Chelonia 
The (10) dorsal vertebrae are without transverse or articular processes, 
and along with the ribs are for the most part immovably fused in the 
carapace. The tail and neck are the only flexible regions. There are 
two sacral vertebrae. 
The greater part of the dorsal shield is due to a coalescence of eight 
ribs with eight costal plates derived from the dermis, 
