CARAPACE AND PLASTRON OF THE CHELONIAN REPTILES. 
159 
The youngest Chelonian which I have had the opportunity of examining has been 
the embryo of the common Turtle {Chelone My das), not quite an inch in length 
(PI. XIII. figs. 1 , 2 , 3). At this period the broadest part of the animal is the head, 
across the large prominent eye-balls. The neck is shorter than the head, the cara- 
pace is a long narrow ellipsoid, more convex than in the adult, defined by a feebly- 
indicated, thickened border : the region of the plastron (fig. 3j is flatter, perforated 
by the large vitelline duct and vessels (u). The scapular arch (fig. 2 a, si, 52 ) divides the 
base of the neck from the fore-part of the carapace and plastron, and the anterior 
and posterior limbs present the simple form of undivided paddles, which they after- 
wards retain in this and other marine species. Although the ribs (fig. 1 a, 1 to ho) 
are visible through the integument of the back, and the slender entosternum (fig. 3 a, s) 
and two transverse linear rudiments of the plastron {hs and ps), on each side, are more 
obscurely seen beneath the integument of the abdomen, yet the corium covering 
these parts is thicker, and its texture denser than in the embryo of the lizard or that 
of the fowl of corresponding size and development ; the general resemblance in the 
form of the body being very close at this period, to the bird, by reason of the nor- 
mal proportions of the trunk and the shortness of the tail. The most advanced 
parts of the osseous system are plainly those which belong to the endo-skeleton, and 
which at this period deviate comparatively little from the normal type. As my pre- 
sent object relates to the thoracic-abdominal case, I shall confine my remarks chiefly 
to that part of the skeleton. 
Ossific matter has begun to be deposited in the cartilaginous foundations of the 
neurapophyses (figs.wi — wio, 1 a, 2 a), and of the pleurapophyses {di — 1 10 ), but not 
in the neural spines or the centrums. 
Ten pairs of pleurapophyses (dorsal or vertebral ribs) have been established, much 
more nearly equal at the present than at a subsequent period ; the first {d 1 ) and the 
two last (c ?9 and ho) being the shortest: all of them are simple, slender, cylindrical, 
slightly bent towards the ventral surface, terminating freely near the thickened bor- 
der of the dermal basis of the carapace. The scapulae (ih.bl) closely resemble the 
other pleurapophyses : it is impossible to mistake their general homology as the same 
elements of the vertebral segment : they are equally simple and cylindrical, and their 
ossification has made the same progress : but their position is more nearly vertical, 
with the upper end abutting against the fore-part of the first thoracic rib (c^l), and 
the lower end bent inwards towards the entosternum {s ) ; the position is very simi- 
lar to that which the scapula presents in the correspondingly developed embryo of 
the bird, in which, by a subsequent movement of backward rotation, the slender rib- 
like scapula comes to overlap the anterior thoracic ribs : but the primitive vertical 
position — the more normal position in relation to the archetypal skeleton — is retained 
throughout life in the Chelonia as in the Monotremata. In the region of the plastron 
the entosternum is represented by a slender median cartilage, pointed behind (fig. 
3 a, 5 ) the hyosternals {hs) by a pair of transverse cartilages, commencing near the 
