SIDE-NECKED TORTOISES. 



93 



Nocturnal and carnivorous in their habits, the snake-necked tortoises appear 

 to agree in their general mode of life with the majority of fresh-water species. 

 During the daytime they are generally to be found lying asleep on some dry spot 

 near the water, with the neck bent on one side, and the head, like the limbs and 

 tail, retracted within the margins of the carapace. When disturbed, the head and 

 neck are, however, shot out with marvellous rapidity, reminding the observer of the 

 sudden dart of a snake. 



Australian Long- In Australia and New Guinea the place of the preceding group 



Necked Tortoises. i s taken by another genus of long-necked tortoises, technically known 

 as Chelodina, the members of which may be recognised by the presence of a 

 normally placed nuchal shield 

 on the carapace, coupled with 

 the circumstance that the inter- 

 gular shield of the plastron, 

 instead of being placed between 

 the gulars, as in the figure on 

 p. 89, is situated behind the 

 latter, which consequently meet 

 in the middle line. The ver- 

 tebral horny shields are longer 

 than broad, and the whole of 

 the shields remarkable for their 

 extreme thinness. On remov- 

 ing the latter from the carapace, 

 it will be found that, owing to 

 the absence of neural bones 

 all the pairs of costal bones 

 meet in the middle line, — a peculiarity shared with one American, and two other 

 Australian genera of the family. There- are four species of these long-necked 

 tortoises, three of which are found in Australia, while the fourth is Papuan. 



In addition to the foregoing, there are four other genera 

 belonging to the family under consideration, collectively distinguished 

 by their shorter necks, the length of which is inferior to that of the back. Of 

 these the American Rhinemys, Hydrastis, and Platemys are characterised by the 

 narrow anterior extremity of the lower jaw, and by the first vertebral shield of 

 the carapace being wider than either of the others. The second of these genera, of 

 which a member is represented in the accompanying figure, is by far the most 

 numerous in species; and is noteworthy on account of being represented by a 

 fossil species in the Eocene deposits of India. The third genus differs from the 

 other two in the absence of neural bones to the carapace. On the other hand, the 

 two Australian genera — Emydura and Elseya, — both of which present the feature 

 last mentioned, are distinguished by the broad anterior extremity of the lower 

 jaw, and by the first vertebral shield of the carapace not exceeding the others 

 in size. 



Oreaved The tortoises which may be conveniently designated by a 



Tortoises, translation of their scientific title (Podocnemis) — so named on account 



AN AMERICAN SIDE-NECKED TORTOISE (Hydmspis). 



Other Genera. 



