EXTINCT REPTILES. 



487 



DIFFERENCES BETWEEN CROCODILES, ALLIGATORS, 

 AND GAVIALS. 



Alligators. 



The head is short and 

 broad. 



First and fourth lower 

 teeth bite into pits in the 

 upper jaw. 



The union of the two rami 

 of the lower jaw does not ex- 

 tend beyond the fifth tooth. 



The nasal bones form part 

 of the nasal aperture. 



The teeth are very un- 

 equal. 



The scutes on the neck are 

 distinct from those on the 

 bnck. 



All American, except one 

 Chinese species. 



Crocodiles. 



Longer. 



The first bites into a 

 pit ; the fourth into a 

 groove. 



Not beyond the 

 eighth. 



As in the alligator. 



Unequal. 



Sometimes distinct, 

 sometimes continuous. 



Living in Africa, 

 India, Australia, 



Cuba, S. America. 



Gavials. 



The snout is very long. 



First and fourth lower 

 teeth bite into grooves in the 

 upper jaw. 



The union extends at least 

 to the fourteenth. 



The nasal bonesdonot form 

 part of the nasal aperture. 

 Almost equal. 



Continuous. 



Living in India, Borneo, 

 N. Australia. 



History of Crocodilians. — These giant reptiles form a decadent 

 order. Fossil forms are found in Triassic strata {e.g.y Belodon, Para- 

 suchus, and Stagonolepis) ; their remains are abundant in Jurassic rocks. 

 In Oetaceous strata, crocodilians with procoelous vertebrae first occur, 

 the pre-Cretaceous forms having centra of the amphicoelous type. 

 Huxley has worked out an " almost unbroken " series from the ancient 

 Triassic crocodilians down to those of to-day. 



Extinct Orders of Reptiles. 



Reptilian fossils occur in Permian strata and thence onwards ; most 

 of the orders are represented in the Trias ; the golden age of Reptiles 

 was during Jurassic and Cretaceous ages. 



Some of the modern Reptiles are indissolubly linked to very ancient 

 progenitors, the Crocodilians of to-day to those of the Trias, the New 

 Zealand "lizard" to the Triassic Rhynchocephalia, but no Reptilian 

 genus has persisted age after age as Ceratodus has done among Fishes. 



On the other hand, many types once abundant have long since dis- 

 appeared. These extinct orders we shall now mention. 



Order Anomodontia. _ Reptiles represented in the Permian and the 

 Trias, with lizard-like bodies and legs adapted for walking. They are 

 believed to be related to the Labyrinthodont Amphibians, and to the 

 ancestors of Mammals. Examples : — Pariasaurus^ Galesaurus^ Dicyn- 

 odon. 



Order Sauropterygia. Reptiles represented from the Trias to the 

 Chalk, without exoskeleton, usually with a long neck and a short tail. 



