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skeletal parts of the incipient bones as does the carapace : the triangular entosternal bone, 

 the greater part of the episternals and xiphisteraals, and a smaller proportion of the hyo- 

 sternals and hyposternals, are compact bone with a smooth shining free surface : the greater 

 part of the broad hyostemal and hyposternal plates, the entire and even margins of which 

 are encroaching on the central unossified space of the plastron, are subgranular, coarser and 

 more opake than the slender endo-skeletal parts, which still retain much of the primitive rib- 

 like form they present 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. From the flattened and expanded 

 inner and lower end of the hyosternal 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. As the inner and lower toothed border of the endo-skeletal part 

 of the hyosternal touches the oute'r border of the entosternal bone, the haemal arch of the 

 first segment of the thoracic-abdominal case (second vertebra of the back) is completed inde- 

 pendently of the marginal pieces ; and, in point of fact, the third and fourth marginal plates 

 are simply applied to the outer side of the hyosternal where it bends upwards to join the first 

 long pleurapophysis or rib of the carapace. The most obvious and 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 

 Vertebrata, is that the centrum, the neural arch, and the pleurapophysis being unquestion- 

 ably the elements so called, the hyosternals are the ' heemapophyses ' (sternal ribs or costal 

 cartilages), and the entosternum 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, almost filling the interspace, for one-fourth of its length, between 

 that rib and the next, and thus again forming the hsemal arch of the segment without the 

 intervention or aid of any of the marginal plates, the seventh of these being simply applied 

 to the outside of the hyposternal, where its slender elongated extremity bends upwards to 

 join the vertebral rib : and the only incomplete part of the arch is the unossified median 

 space between the lower expanded and dentated ends of the hyposternals, between which the 

 entosternal, or true sternal piece, does not extend backwards. So that the condition of this 

 fifth segment of the thoracic-abdominal box, in the young Tortoise, repeats that of u poste- 

 rior dorsal segment of a mammal or crocodile, in which the cartilages of the ribs, or abdo- 

 minal ribs, do not reach the sternum ; and the Ornithorhynchus offers a special resemblance 

 to the Tortoise in the expansion of the semiossified heemapophyses, or cartilages of its ' false 

 ribs.' The xiphisternals, viewed in like manner as 'heemapophyses,' repeat the condition 

 of those abdominal ones in the Crocodile and Plesiosaur which do not ascend so high as to 

 join their pleurapophyses or vertebral ribs. The difference between the endo-skeletal and 

 exo-skeletal portions of these elements of the plastron is as plain, and the contrast, indeed, is 

 almost as great, in the young Tortoise as in the adult Trionyx, where the superadded ossifi- 

 cation, at the expense of the dermal system, is characterized by the vermicular or punctate 



