CARAPACE AND PLASTRON OF THE CHELONIAN REPTILES. 
163 
with the pleurapophyses, which do not yet reach them : their ossification has been go- 
verned by the presence of the marginal epidermal scutes, and, as in the case of the cos- 
tal plates, by the points of junction of contiguous scutes ; each marginal ossification is 
accordingly impressed by the lines indicating the junction of the marginal epidermal 
scutes with each other and, in the case of the middle ones, with the contiguous scutes 
of the plastron. The number of the marginal plates accords, moreover, with that of 
the marginal epidermal scutella, not with that of the ribs, 
The plastron of the immature Tortoise (figs. 6 & 7) presents the same difference 
in the texture and surface of the endo-skeletal and exo-skeletal parts of the incipient 
bones as does the carapace : the triangular entosternal bone (.s), the greater part of 
the episternals {es) and xiphisternals {xs), and a smaller proportion of the hyosternals 
{hs) and hyposternals {ips), are compact bone with a smooth shining free surface ; the 
greater part of the broad hyosternal and hyposternal plates, the entire and even mar- 
gins of which are encroaching on the central unossified space of the plastron, are sub- 
granular, coarser and more opake than the slender endo-skeletal parts, which still 
retain much of the primitive rib-like form they presented in the foetal Chelone, and 
are seen applied, as it were, to the inner (upper) surface of those dermal plates. The 
median extremities of the true endo-skeletal parts have begun to expand, and to shoot 
out the pointed rays of tooth-like processes which they retain in the Trionyces and 
the marine Chelonia (fig. 3). From the flattened and expanded inner and lower end 
of the hyosternal (fig. 7, hs) the main body of the bone arises and curves upwards, 
outwards and forwards, in the form of a long and slender rib, and applies itself to the 
inner and fore part of the first elongated pleurapophysis of the carapace, extending as 
far as the incipient dermo-costal plate ; the rib-like part is represented detached from 
the rest of the hyosternal in fig. 5, hs. As the inner and lower toothed border of the 
endo-skeletal part of the hyosternal touches the outer border of the entosternal bone, 
the haemal arch of the first segment of the thoracic-abdominal case (second vertebra 
of the back) is completed independently of the marginal pieces ; and, in point of fact, 
the third and fourth marginal plates (fig. 8, m) are simply applied to the outer side of 
the hyosternal {h) where it bends upwards to join the first long pleurapophysis {pi) or 
rib of the carapace. The most obvious, and, I believe, the most natural explanation 
of this first complete segment of the thoracic-abdominal region of the young Tortoise, 
according to the typical vertebra, and the composition of the corresponding segment in 
the nearest allied Vertehrata, is — that the centrum (PI. XIII. fig. 8, c), the neural arch 
(ns), and the pleurapophysis (j»/),are the parts so indicated by the initial letters ; that 
the hyosternals (h) are the hsemapophyses (sternal ribs or costal cartilages), and the 
entosternum {hs,s) is the ‘haemal spine’ or sternum proper. The hyposternals in the 
young Testudo resemble the hyosternals, but are shorter ; the slender rib-like portion 
which curves upwards and outwards applies itself to the back part of the extremity 
of the fifth rib of the carapace {fig.b^ps), almost filling the interspace, for one-fourth 
of its length, between that rib and the next, and thus again forming the haemal arch 
Y 2 
