28 TORTOISES. 



LESSON II. 



ORDER OF CHELONIANS, or TORTOISES. Organization. Division 

 into four Families. Land Tortoises. Fresh water Tortoises. 

 River Tortoises. Sea Tortoises. Tortoise Shell: its use. 



ORDER OF CHELONIA. 



Fig. 2. ii Fig. 3. 



GREEK TORTOISE. 



1. Tortoises are recognised at first sight by the curious defen- 

 sive armour with which nature has provided them. A double 

 shield envelopes all parts of their body, except the head and neck, 

 the four paws and tail, which in general, may be also concealed 

 in this species of solid box. 



2. When studying the mammalia, we mentioned some animals 

 that have the body protected by a sort of shell. The Armadillo 

 furnished an example ; but that armour, formed only of aggluti- 

 nated hair, or by a particular modification of the epidermic layers 

 of the skin, in no wise resembles the double shield of the Tor- 

 toises; for the latter is composed of bones, which are widened, 

 and intimately united to each other ; the carapax, (2.) or upper 

 shell is formed by the union of the ribs and dorsal vertebra? ; the 

 plastrum, (3.) or lower shell, is the sternum. Consequently, 

 these organs are a portion of the skeleton which, instead of being 

 lodged in the substance of the soft parts, has become superficial, 

 and is only covered by a thin, dry skin. 



3. The bony frame of tottoises, to present this unusual dispo- 

 sition, must be, of course, extremely modified : still, we find in 



1. What is the most striking featbre of Tortoises ? 



2. What is the essential constitution of the upper, and of the lower sheil 

 of Tortoises? 



3. How does the skeleton of a Tortoise differ from that of a mammal ? 



