10 GUIDE TO REPTILES AND AMPHIBIANS. 



these Eeptiles. Crocodiles bury their eggs in the sand, where they 

 are hatched by the heat of the sun's rays. 



Four large specimens are exhibited on a stand in the middle of 

 the gallery, and the others in the wall-cases. 



Case 3. In case No. 3, two specimens are placed side by side in order to 



show a notable difference between the skull of a Crocodile and an 

 Alligator (figs. 7 and 8). In the former (14 a) the fourth lower 

 tooth is generally received into a notch on the side of the upper jaw, 

 while in the latter (28 «) it bites into a pit. Crocodiles have also 

 fewer lower teeth than Alligators ; the number in the former varying 

 from 14 to 15, and in the latter from 17 to 22. In most Crocodiles 

 the skull is narrower than in Alligators, with the pits in the temporal 

 region (shown in the specimens in the upper part of the case) larger, 

 but, as mentioned above, some species of the former approximate 

 very closely to the latter in these respects. 



Case 3. Alligators and Caimans are broad-nosed Crocodilians, distin- 



guished from Crocodiles, as stated above, by the fourth lower tooth 

 being generally received into a pit in the upper jaw, and the small 

 size or obliteration of the pits in the temples ; the number of teeth 

 being from 17 to 20 in the upper, and from 17 to 22 in the lower 

 jaw. In the true Alligators the nasal bones divide the aperture of 

 the nostrils, the bony plates on the back are separate, and on the 

 under surface these are either very thin or wanting. In the Caimans, 

 or South American Alligators, on the other hand, the aperture of the 

 nostrils is not divided by the nasal bones, the bony plates of the back 

 are articulated together, and a full series of similar plates occurs on 

 the lower surface of the body. 



Of true Alligators, one {Alligator mississippiensis, 31, fig. 10) is 

 North American and the other {A. sinensis, 32) Chinese — a distri- 

 bution explained by the occurrence of allied forms in the Tertiary 

 deposits of Europe. The Chinese species alone has thin bony plates on 

 the under surface. Both kinds inhabit swamps. The female of the 

 North American Alligator constructs a large nest, in which the eggs 

 are deposited in layers. Some species of Caiman, which may reach 

 20 feet in length, make regular migrations, retreating to the flooded 

 forests in the wet season, and returning to the rivers during the dry 

 months. In some districts they are called Jacares. 



The Caimans (25-27) are peculiar in possessing a shield of 

 bony plates in the skin of the under side of the body. On the 

 under surface each plate consists of two distinct pieces, united by a 

 transverse suture. In the species of which this armour is exhibited, 



