458 CHELONIANS. 



external, continued from the common skin of the head, and vary, 

 ing as to form in different genera ; one, internal, resembling the 

 nictitating membrane of birds ; and moved by muscles appropri- 

 ate to that office. The form of the pupil is round, as in birds. 

 The iris is always colored, usually dark, but in some, red, or 

 even milk-white. 



Tortoises have no movable external ear, but in all are found 

 the tympanic orifice and membrane which are wanting in Suuri- 

 ans, and the sense of hearing is consequently well developed. 

 The sense of smell appears to be at a low degree. The nostrils 

 open on the most anterior part of the upper jaw or mandible, and 

 are close to each other. In the River Tortoises, and in the 

 MATAMATA, a Marsh Tortoise, of South America, the nostrils are 

 prolonged into a sort of flexible proboscis, which the animals can 

 raise for the purpose of respiration, between the large, floating 

 leaves of water plants, while they lurk with their bodies con- 

 cealed below them, and immersed in the water, lying in wait 

 for their prey. 



The males of the Tortoises are, in general, smaller than the 

 females, and commonly distinguished by the plastron, which is 

 slightly concave. Tortoises have a voice, that is, they have, 

 more or less, the faculty of uttering distinct sounds. They vary 

 in their food according to the localities which they are accus- 

 tomed to frequent. Some live on marine plants ; others on small 

 animals, in addition to vegetable food. They require but little 

 nourishment, and can pass months, and even years, without eat- 

 ing. Turtles, in their growth, are exceedingly slow, coming to 

 maturity the latest of all the Reptiles. But, at the same time,, 

 they are very long-lived. Land Tortoises have been known to 

 live one hundred and twenty years, and some have even reached 

 more than two hundred years. 



According to Agassiz, their eggs, up to the seventh year, are 

 of small size, numerous, yet not distinguishable into sets; but 

 with every succeeding year, there appears a larger and larger 

 set of eggs; each set being made up of the usual number which 

 the species lays, so that a Turtle of eleven years old, for the first 

 time, contains mature eggs ready to be laid in the spring. The 

 larger eggs always appear in regular sets, of* a definite number, 

 and these coincide with the number laid by that particular species 

 at one time. Four sets can be readily distinguished ; one of 

 them mature eggs; another about half the size; a third still 

 smaller; and the fourth smaller still, (about the size of a large 

 pin's head ;) below these, it is difficult to distinguish the differ- 

 ence in size. "Turtles," says Agassiz, "lay once a year; 



