CROCODILES, ALLIGATORS, G A VIALS. 



589 



(c] The gavials or gharials, of the genus Gavialis, are distinguished by 

 their long narrow snout. In the Ganges and its tributaries, G. gangeticus, 

 said to attain a length of twenty feet, is common. They feed chiefly on 

 fishes. "Old males have a large cartilaginous hump on the extremity 

 of the snout, containing a small cavity for the retention of air, by which 

 means these individuals are enabled to remain under water for a longer 

 time than females or young." 



DIFFERENCES BETWEEN CROCODILES, ALLIGATORS, 

 AND GAVIALS. 



History of Crocodilian*. These giant reptiles form a decadent order. 

 Fossil forms are found in Triassic strata (e.g., Belodon, Parasuchus, 

 and Stagonolepis] ; their remains are abundant in Jurassic rocks. In 

 Cretaceous strata, crocodilians with proccelous vertebras first occur, the 

 pre-Cretaceous forms having centra of the amphiccelous type. Huxley 

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

 crocodilians down to those of to-day. 



Development of Reptiles. 



As the development of Birds will be discussed in the next chapter, a 

 few notes on that of Reptiles, which is in many respects similar, will 

 be sufficient. 



The ovum contains much yolk, at one pole of which there is a small 

 quantity of formative protoplasm surrounding the germinal vesicle. 

 Formation of polar globules has not been observed. The segmentation 

 is necessarily meroblastic and discoidal, as in Birds. 



The segmented area or blastoderm, originally at one pole, gradually 

 grows round the yolk. The central region of the dorsal blastoderm 

 is separated from the yolk by a shallow space filled with fluid, and 

 is clearer than the rest of the blastoderm. In this central region or area 

 pellucida, the germinal layers and subsequently the parts of the embryo 

 are established, while the rest of the blastoderm the area opaca 



