TURTLES, TERRAPINS, AND TORTOISES 37 



retractile within the shell. It embraces three closely allied 

 genera, namely, Sternothcerus^ Pdomedusa, and Podocnemis, 

 all represented in Madagascar ; the two first also inhabit 

 tropical and South Africa, while the last is represented 

 in South America as well. 



The front lobe of the plastron is movable in Sterno- 

 thcerus ; Pelomedusa and Podocnemis, in which this is not 

 the case, are to be distinguished by the fact that in the 

 former, in common with Sternothcsrus, both pair of limbs 

 are provided with five claws, while, in the latter, the hind 

 limbs possess four only, as usual in tortoises and terrapins. 



The twenty species belonging to Sternothcerus are all 

 very aquatic, and seldom leave the water, while Pelomedusa 

 galeata, characterized by a flat, roundish shell, the sole 

 representative of its genus, spends a considerable part of 

 its existence on land. 



The Amazon Terrapin, Podoenemis expansa, of the 

 rivers of tropical America, attains a length of two and 

 a half feet ; it is the best known of the eight members 

 of the genus, owing to the great local commercial 

 value of the oil produced from its eggs, of which enor- 

 mous quantities are collected at the breeding season. 

 According to Bates, who gives a lengthy account of 

 the breeding habits and life history of this species in 

 his book. The Naturalist on the Amazon, some 6,000 jars, 

 each holding three gallons of oil, are annually exported 

 from the Upper Amazons, and about 2,000 more jarsfuU 

 are consumed by the inhabitants, the total number of 

 eggs destroyed annually amounting to about forty-eight 

 millions. The eggs, which are deposited at night in deep 

 holes, previously scraped out in the ground on the banks of 



