THE SIDE-NECKED TORTOISES 



2451 



Other Genera 



latter, which consequently meet in the middle line. The vertebral horny shields 

 are longer than broad, and the whole of the shields remarkable for their extreme 

 thinness. On removing 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, Hydraspis, 

 and Platemys are characterized 

 by the narrow anterior extrem- 

 ity of the lower jaw, and by the 

 first vertebral shield of the cara- 

 pace being wider than either of 

 the others. The second of these 

 genera, of which a member is 

 represented in the accompany- 

 ing 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. AN AMERICAN SIDE-NECKED TORTOISE (Hydraspis}. 

 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. 



The tortoises which may be conveniently designated by a transla- 

 tion of their scientific title (Podocnemis} so named on account of the 

 presence of a pair of large shields on the outer side of the hind-foot of 

 the typical species bring us to the second family of the group under consideration. 

 This family (Pelomedusidtz} , which contains three genera, and is now confined to 

 Africa, Madagascar, and South America, is broadly distinguished from the last by 

 having eleven elements in the plastron, owing to the presence of a pair of mesoplas- 

 tral bones; while the neck is completely retractible within the margins of the shell. 

 The skull differs from that of the preceding family in having a bony temporal arch, 

 as shown in the figure on p. 2446; while it lacks the distinct nasal bones generally 

 found in the former. 



The largest and best-known representative of the whole family is the giant 

 Amazonian tortoise (Podocnemis expansa), which considerably exceeds in size all 

 other members of the entire group, having a shell which may measure as much as 



