50 



KNOWLEDGE 



[January 1, 1890. 



first or gnlar {<iii i shields of the plastron. This additional, 

 or intertiular, shield, as it is termed, is very frequently of 

 a battledore shape, as in Fig. 4. 



In regard to the bones composing the upper shell, or 

 carapace, it will be observed, from Fig. 2, that although 

 they by no means correspond in contour with the over- 

 lying horny shields, yet that a decided general similarity of 

 arrangement obtains between the two. Thus there is a 

 single middle row of bones corresponding to the middle 

 row of homy shields, although the bones are smaller and 

 more numerous than the shields. Similarly there is a 

 series of lateral bones on either side of the middle row ; 

 while m like manner the border of the carapace is formed 



Fig. 4. — The Frost Portion of the Carapace of a Freshwater- 

 Tortoise OF the SorTHER>- HEMISPHERE, WITH THE HORST 

 Shields removed. The thick lines indicate the boundaries of the 

 homy shields. 



of a series of marginal bones corresponding very closely 

 with the marginal homy shields. The middle series of 

 bones of the carapace are severally attached to the summits 

 of the spines of the back-bone or vertebrfB ; while the 

 lateral bones are nothing more than large expanded plates 

 l}-ing on the ribs, to which they are so completely welded 

 that they are generally regarded as part of the same bone. 



From this very brief glance at the structure of the 

 Chelonian sheU, we are now enabled to understand one of 

 the most pecuUar features of the organisation of these 

 remarkable reptiles. This peculiar feature is, that whereas 

 the shoulder-blade in all other animals lies entirely on the 

 outer side of the ribs, in the tortoises and ttu-tles it is 

 situated within the ca^^ty enclosed by the ribs and shell. 

 Similarly the haunch-bones, which in all other animals 

 lie close to the outer surface of the body, are likewise 

 shifted ^rithin the cavity of the ribs and shell. To reach 

 the shoulder-blades and haimch-bones the bones of the 

 arm and leg are bent in a manner quite peculiar to this 

 group of reptiles. 



One other peculiarity in the bony structure of the gi-oup 

 still remains to be mentioned. It is probably weU known 

 to all our readers that ordinary reptiles — such as crocodiles 

 and lizards — are furnished -with well developed teeth, 

 which are fr-equently of great size. In all Chelonians, on 

 the contrary, teeth are totally wanting, and their fimction 

 is consequently performed by the margins of the jaws, 

 which form sharp cutting edges, and are ensheathed with 

 a coating of thick horn. This total absence of teeth is 

 well shown in Fig. 1, and still better in Fig. 5, where the 

 bones of the skuU are represented with the flesh and 

 skin removed. Aaiother remarkable peculiarity of the 

 Chelonian skuU is foimd in the circumstance that the two 

 sides of the lower jaw are finnly united together by bone 

 at the chin, instead of being more or less completely 

 separated, as in aU other li\-ing reptiles. A thii-d peculiarity 

 is the development of the hinder extremity of the skiiU 

 into a long spike-like process (Fig. 5, o), which gives a 

 very characteristic contoiu- to this part. The whole of the 



skuU is covered over wth horny shields (Fig. 1) in the 

 living animal ; so that in this respect the structure of the 

 skull corresponds exactly with that of the shell. 



Jjn the entire absence of teeth, coupled with the horny 

 sheathing of the jaw, and the solid imion of the two bones 

 of the lower jaw at the chin, tortoises agree with birds. 

 Many, or all, of the birds of the Secondary geological epoch 

 are, however, now kno^vTi to have been provided with a com- 

 plete set of teeth, and it is therefore highly probable that 

 we shall some day find the fossil remains of extinct 

 tortoises which were also furnished with these useful 

 implements, since there appears to have been a tendency 

 in many groups of animals, and more especially in birds 

 and their near relations the reptiles, to lose their teeth. 

 Thus the Fish-lizards of the Cretaceous rocks of the United 

 States are all characterised by the absence of teeth ; and 

 a similar condition obtains in the Pterodactyles, or flying 

 reptiles of the same deposits, by which feature they are 

 widely distinguished from their Old World aUies. 



All the fossil tortoises and turtles at present known to 

 us agree with existing types in their absence of teeth, as 

 well as in the general characters of the shell ; and we have 

 at present (on the assumption that some mode of evolution 

 is the true explanation of the mutual relationships of the 

 different gi-oups of animals) no evidence to connect the 

 Chelonians very closely \nih any other type of reptiles. 

 It is, however, very probable that the bony plastron of 

 the tortoises is an extreme development of the peculiar 

 system of so-called abdominal ribs found on the lower 

 surface of the body in crocodiles and many extinct groups 

 of reptiles. 



Chelonians vary greatly in their habits, and in the 

 nature of their food. Thus the land-tortoises, as their 

 name imphes, are terrestrial, although all of them can 

 swim ; while some closely allied types are aquatic. The 

 feet of the former are pro\"ided with strong claws, and 

 their food consists exclusively of vegetables, of which they 

 consiune a vast amount. The pond-tortoise of Southern 

 Europe, which, together with the common Grecian land- 



Fig. 5. — Left Side of the Skdll of a Tortoise, vma the 

 Lower Jaw Displaced. «i,^»i, upper jaw; s, cheekbone : t, cavity 

 of ear ; f, pf, bones over the eye and nose-cavity ; p. hollow of 

 the tompies ; o. posterior spine. 



tortoise, is the only Eiu-opean representative of the gi'oup, 

 has, however, the feet webbed, and subsists on animal 

 substances, such as woims. Many of the Indian water- 

 tortoises are also caiTiivorous ; this being especially the 

 case with the weU-known soft tortoises of the rivers of 

 the warmer parts of the globe. In these forms the shell 

 is improvided with horny shields, but has the outer sur- 

 face of the bones ornamented with a beautiful net-like 

 sculpture, and merely covered with a thin skin. In the 

 turtles, again, which are of marine habits, although they 



