3S4 REPTILIA—ORDER IL—CHELONIA. 



and anal shields ; while in some cases an unpaired intergular is interposed 

 between the two gulars. Certain small shields on the borders of the slits for 

 the fore and hind limbs need not be further referred to. 



If we now strip off the horny shields, we shall find that the underlying 

 bony plates conform to a great extent with the arrangement of the former 

 although presenting certain well-marked differences. In the middle line of 

 the upper shell runs a single series of small many-angled plates termed the 

 neurals, all attached below to the back-bone. A larger unattached nuchal 

 plate in front, and one or two similarly detached pygals behind, complete this 

 median unpaired series. The sides of the carapace are formed by the eight 

 pairs of costal plates, which are firmly welded to the underlying ribs ; while 

 its borders are composed of the marginal plates, which resemble the corre- 

 sponding horny shields in being angulated, and some of which join the 

 plastron, when the latter is fully developed. Exclusive of these marginals, 

 the lower shell or plastron generally consists of one unpaired and four paired 

 elements. The front pair, corresponding to the collar-bones of other verte- 

 brates, are the epiplastrals, behind or between which is the single ento- 

 plastral. Then follow the hyoj)lastrals, hypoplastrals, and xiphiplastrals ; 

 the latter forming the hinder extremity of the lower shell, and being fre- 

 quently more or less deeply notched in the middle line. In addition to 

 these elements, a few tortoises have a pair of mesoplastral bones intercalated 

 between the hyo- and hypoplastrals, thus bringing up the total number of 

 bones in the lower shell to eleven; 



Scarcely less peculiar than their shells are the jaws of the Chelonia, which 

 are invariably cased in horny sheaths, with sharp cutting edges, like the 

 beaks of birds, and show no vestiges of teeth at any period of life, Gener- 

 ally the head is relatively short and wide ; but there is a considerable amount 

 of diversity in this respect, as there is in regard to the structure of the skull. 

 In all cases, however, the quadrate-bone is firmly fixed between the adjacent 

 bones of the skull, to which its relations are somewhat different from those 

 obtaining in the Crocodilia. There are many other peculiarities connected 

 with the bony framework of these reptiles ; but those mentioned are amply 

 sufficient to distinguish them from all other groups. 



The order is divided into three sub-orders, respectively known as the 

 Cryptodira, Pleurodira, and Trionychoidea ; and these divisions are important, 

 not only on account of indicating structural differences, but also on distribu- 

 tional grounds. 'J bus, whereas the first group attains its maximum develop- 

 ment in the countries to the north of the Equator, the second group is now 

 entirely restricted to the Southern Hemisphere, and is the only one repre- 

 sented in Australia. No members of the order are found in New Zealand ; 

 and the third group is unknown in both Australia and South America, as 

 indeed it is in Europe, although this is probably duo to temperature. 



Great differences are observable in the mode of life of the different 

 members of the order, the typical tortoises being almost or quite exclusively 

 terrestrial, the soft tortoises fresh-water, and the turtles marine. All are, 

 however, fond of water, and even the most exclusively terrestrial species are 

 fond of an occasional bath. Many of the so-called terrapins maybe regarded 

 as amphibious creatures, spending much of their time in the water, but often 

 dwelling for a considerable period on dry land. Between the land tortoises 

 and the ordinary fresh-water terrapins there is not much difference in the 

 structure of the feet ; but in the turtles these are modified into paddles, 

 adapted solely for swinniiing, and forming poor oi-gaus of progression on 



