272 



AN INTRODUCTION TO ZOOLOGY. 



Fi^lOl. — The common Tortoise, 

 3 iutaria (or europasa) . 



The Chelonia include the 

 TurtleSj and several groups of 

 Tortoises. They have no teeth, 

 but a beak somewhat resem- 

 bling that of a bird. The 

 limbs are usually more or less 

 adapted for swimming as well 

 as for walking. The head and 

 limbs can usually be retracted 

 under the shell. The back of 

 this shell is called the cara- 

 pace, and the front is called 

 the plastron. 



BlEDS (AVES). 



Liiuks between Birds and Reptiles. — Strange 

 as it seems at first sight, the slow and cold-blooded 

 reptile is closely related to the birds, the most active 

 and consequently the most warm-blooded of the Ver- 

 tebrates. This is shown, not only by the similarity of 

 their structure in many respects, but by the existence 

 of links between the two. One of these links is 

 afforded by the Odontornithes, fossil birds which have 

 teeth, instead of possessing a toothless beak, like other 

 birds. Another link is afforded by a fossil bird called 

 Archceopteryx, which possessed teeth, and had more- 

 over a long tail with separate vertebras, and corre- 

 sponding successive pairs of feathers, instead of a 

 modified tail with fused vertebrEE and crowded 



