152 PROFESSOR OWEN ON THE DEVELOPMENT AND HOMOLOGIES OF THE 
‘ median ’ series of symmetrical plates {ch, « i to ^ n), and of two ‘ lateral ’ series form- 
ing a pair {pi i to pi s), the whole being surrounded by a circle of ‘ marginal ’ pieces 
{m 1 to pp), completed anteriorly by ch, the first of the median series. Of the median 
series eight i to .s s) are attached to the spines of eight subjacent vertebrae : the 
lateral or parial plates (pi i to pi s) are attached to, and more or less blended with, 
the ribs of the same vertebrae, and the ends of these ribs usually articulate by gom- 
phosis with a corresponding number of the marginal pieces, of which, however, there 
may be from twenty-four to twenty-six, including the two median and symmetrical 
ones {ch and py). That these marginal pieces are the least essential parts of the 
carapace is shown not only by their inconstant number, but by their partial or total 
absence in some of the soft-turtles {Gymnopus, Sphargis). 
In the present communication the me- 
dian pieces ( 51 — .sn) are called the ‘neural’ 
plates; the lateral pieces (p/ 1 — jo/ 8 )the ‘cos- 
tal’ plates : the term ‘marginal’ is restricted 
to those peripheral pieces which form pairs 
(mi tom 12 ) ; the anterior symmetrical piece 
{ch), constant in all Chelonia, is called the 
‘ nuchal’ plate ; the posterior symmetrical 
piece {py^, which is wanting in all the Trio- 
nycidce,\?, the ‘pygal’ plate. I enumerate the 
neural plates in the order in which they are 
numbered by Bojanus in the Tab. III. and 
IV. of his great work*. The neural arch 
connate with the first neural plate (.si) is sup- 
ported partly by the centrum of the verte- 
bra to which the first pair of free ribs (fig. 2, 
c i) is articulated, and which, therefore, is 
reckoned as the first dorsal vertebra : these 
ribs are small and slender, attached at both their extremities, the outer end abutting 
against the under part of the first pair of costal plates, which they help to sustain. The 
second to the ninth dorsal vertebrae inclusive, being those which are more immediately 
connected with the neural and costal plates, may be called ‘ vertebrae of the carapace:’ 
their characters, though not less artificial than those which distinguish the ‘ dorsal ’ or 
‘ lumbar ’ vertebrae of other reptiles, are much more marked and constant. The eighth 
vertebra of the carapace is succeeded by a vertebra, whicli in some species {e. g. Chelone 
caouanna, fig. 2, 1 1 ) supports a pair of short ribs, in others {Trionyx) none, and which is 
therefore reckoned a ‘lumbar’ vertebra; this is followed by two other vertebrae, with 
short and thickened ribs, abutting against the iliac bones and representing the ‘ sacrum’ 
(fig. 2, ^ 1 ) : as these three vertebrae are not immediately united with the ninth, tenth 
^ Anatomia Testudinis Europsese. Fol. 1819-1821. 
Fig. 2. 
Inner view of the carapace of the Loggerhead Turtle {Chelone caouanna). 
