XV111 
INTRODUCTION, 
excepting that the lines are not interrupted by any undulating or zigzag 
inequalities. The inferior portion of this effectual armour, consists of the 
sternum; and the two portions are more or less intimately connected, in the 
various families, by intervening hones, which must be considered as analogous 
to the cartilages of the ribs in the Mammifera, and which, in fact, in some 
genera, remain, to some extent, permanently cartilaginous. 
The varieties of form and of development which this part assumes, in the 
different groups of tortoises, constitute some of the most constant and essential 
characters by which they are distinguished, and at the same time perfectly 
accord with the structure of the neck, feet, and tail. 
The TusTUDiNiDiE or Land Tortoises, which are exposed to continual attacks 
and dangers, from which their sluggish movements prevent their escape, 
unprovided as they are with any kind of weapons of offence, require defensive 
armour of the most secure kind, and capable of affording the strongest resist¬ 
ance to external injury. They are therefore provided with a hard, solid and 
compact shell, forming an elevated arch, which is calculated to sustain immense 
pressure. In addition to this, the neck and head can be drawn wholly within 
the shell; and the legs-—the solid and clumsy structure of which can only 
permit that slow progress by which these animals are characterized,—are 
admirably adapted, by this veiy peculiarity of form, as well as by the manner 
in which they are articulated, and the hard strong scales which, in most cases, 
protect the anterior portion of them, to defend the more important and vital 
parts within. Thus, when the animal is at rest, the head and neck are wholly 
concealed, and the legs entirely occupy the anterior opening between the 
upper and lower parts of the shell, which opening, in this family, is small and 
contracted, consistently with the circumscribed extent of motion required by 
the feet. 
In the family of the Emydid^i, or those fresh-water tortoises which approach 
most nearly in their structure to the foregoing group, the shell is much flatter, 
and both the anterior and posterior openings between the dorsal and pectoral 
shields are much more expanded, in order to allow of the more free and ex¬ 
tensive motion of the limbs in swimming. The feet also are more slender and 
moveable; and the toes, instead of being contracted, as in the former case, into 
