226 VERTEBRATE ZOOLOGY 



Numbers 9 (b), 10, 12, and 13 separate them from the birds and 



mammals. 

 Numbers 3, 8, and 1 1 separate them from the mammals. 



ANATOMY OF A MODERN REPTILE (TURTLE) 



For several reasons the turtle or tortoise is chosen as a reptilian 

 type for detailed description rather than a lizard, although the latter 

 is in most respects a more representative form, and considerably less 

 specialized than are any of the chelonians. The first reason for our 

 choice is one of expediency, for the turtles are the most plentiful 

 reptile in by far the greater part of the United States. A second rea- 

 son is that they are of convenient size for laboratory work and are of 

 compact structure. There is, however, a third and more fundamental 

 advantage gained by the use of the turtle; it is structurally more nearly 

 related to the group of reptiles from which the mammals are believed 

 to have arisen than is any other living reptile. 



EXTERNAL CHARACTERS 



The turtle is a reptile in a box. This box, whether it forms a com- 

 plete or only a partial housing for the body, head, limbs and tail, has 

 a dome-shaped roof, called the carapace and a flat floor called a plas- 

 tron. Paired lateral pillars join the floor to the roof. The box is open 

 broadly in front and behind in order to allow the head, legs, and tail 

 to emerge; b\it these appendages can all be withdrawn within the 

 shelter of the eaves, and in some cases the front and rear sections of 

 the floor (plastron) are hinged in such a way that they can bend 

 upward and completely close the house after the appendages have 

 been drawn in. The head is of moderate size and somewhat flat; the 

 neck is characteristically long and flexible and capable of being 

 folded up when the head is withdrawn; the mouth is large and tooth- 

 less, but is provided with a sharp-edged, horny beak. The external 

 nares (nostrils) are close together near the end of the snout, sometimes 

 protruding into a regular proboscis. The eyes are situated laterally 

 and have three eyelids: a short opaque upper lid, a longer lower lid 

 which makes the turtle shut its eye upwards instead of downwards as 

 a man does, and a third eyelid or transparent nictitating membrane 

 which may be drawn across the eye from the inner corner. The 

 tympanic membrane is quite similar to that of the frog a 'id is just back 

 of the gape of the jaws. The feet are pentadactyl and each finger is 



