390 CHELONIA CHAP. 



ischiadic plastral ankylosis. Pleurosternum, of the English and 

 Continental Purbeck beds, has meso-plastral plates like the recent 

 Pelomedusidae. Rhinoclielys, of the Cambridge Greensand, has 

 a broad parieto-postfrontal roof, and large nasal bones. Forms 

 like Podocnemis, now restricted to South America, occur in 

 the Eocene of Europe. One of the most aberrant Chelonians 

 is Miolania, from the Plistocene of Queensland and from Lord 

 Howe's Island, remarkable for its huge size and the thick 

 armour on the head and tail ; the head especially carries large 

 paired projections, one pair of which extends horizontally like 

 powerful horns, recalling the queer Theromorphous Elginia. 



We divide the recent Pleurodira into three families, of which 

 that of Carettochelys stands apart by its paddle-shaped limbs 

 and the absence of horny shields. The Pelomedusidae and 

 Chelydidae are closely allied. The former are not Australian, and 

 are externally distinguished by the absence of a nuchal shield. 



Fam. 1. Pelomedusidae. Neck completely retractile within 

 the shell. Carapace without a nuchal shield. The plastron is 

 composed of eleven plates, there being besides the unpaired endo- 

 plastron a pair of meso-plastra, situated between the hyo- and 

 hypo-plastra ; but these meso-plastra meet in the middle line 

 in Sternothaerus only, while in Podocnemis and Pelomedusa they 

 are restricted to small pieces on the bridge, widely separated 

 from each other by the usual hyo- and hypo-plastral suture. A 

 nuchal shield is absent ; there are twenty-four marginal and 

 thirteen plastral shields, inclusive of the conspicuous intergular. 

 The temporal fossa is widely open, except in Podocnemis, where 

 it is partly roofed in by the meeting of the much-expanded 

 quadrato-jugal with the parietal. The palatine bones are in 

 median contact, not separated by the vomer. Nasal bones 

 being absent, the large prefrontals meet in the middle line. 

 The second cervical vertebra is biconvex. 



This family is now represented by only three genera, with 

 about fifteen species in Africa, Madagascar, and South America. 



Sternothaerus. Skull without a bony supratemporal roof. 

 Meso-plastra large, extending right across the plastron. Anterior 

 lobe of the plastron movable, the hinge passing between the 

 hyo- and meso-plastral plates, and between the pectoral and 

 abdominal shields. Fore- and hind-limbs with five short digits 

 and claws. Several species in tropical and southern Africa, and 



