54 GUIDE TO KEPTILES AND AMPHIBIANS. 



rib-like process to the nuchal plate of the carapace ; the entoplastron 

 of the lower shell (as in the Chelydridce) is dagger-shaped. Each 

 flipper has one or two claws. The existing members of the family 

 are marine, but the females come ashore on sandy coasts to lay their 

 spherical eggs. In the edible Green Turtle {Chelone my das, \ 82) the 

 horny shields, of which there are four costal pairs, do not overlap, 

 and there are vacuities between the costal and marginal bones of the 

 carapace. The Hawksbill (G. imbricata, 181, fig. 53), the chief source 

 of commercial " tortoise-shell," is distinguished by the circumstance 

 that, except in old age, the shields of the carapace overlap like slates 

 on a roof. The Loggerhead {Thalassochelys caretta, 179), the largest of 

 all, differs from the others by having at least five pairs of costal horny 

 shields on the carapace, as well as by the obliteration of vacuities in 

 the latter when adult. Of extinct forms, the Eocene and Cretaceous 

 Lytoloma has the secondary bony floor of the palate prolonged back- 

 wards so as to cause the posterior nostrils to open near the occiput ; 

 the symphysis of the lower jaw being also extended backwards. 

 Allopleurum hofmanni, a gigantic species of the Upper Cretaceous, is 

 allied to Chelone in the structure of the shell ; specimens are exhibited 

 in the Geological Department. 



Commercial tortoise-shell of the best quality is yielded only by 

 the Hawksbill ; specimens are exhibited to show this product in its 

 raw state and when polished. 



Sub-order II. — Pleurodira (Side-necked Tortoises). 



The chief distinctive characteristics of this group, which is 

 confined at the present time to the southern hemisphere, are given 

 above on page 41. The most easily seen of these is the manner in 

 which the head is withdrawn into the shell by a lateral movement of 

 the neck, as shown in fig. 56. 



The family Pelomedusidw is typified by the African and Malagasy 

 genus Pelomedusa (210), but also includes the Great Arrau Tortoise, 

 or " Turtle," Podocnemis expcmsa (204), of the Amazons. In all the 

 members of this group the neck is completely retractile within the 

 shell, and the plastron has eleven bones, in consequence of the presence 

 of a pair of mesoplastral elements (fig. 57), which, however, meet in 

 the middle line only in Stemothcerus (212). Podocnemis differs from 

 Pelomedusa by the roofing-over of the temporal region of the skull. 

 The female of the Great Arrau Tortoise is much larger than the 

 male. To the natives of Amazonia this species is of great commercial 



