DEVELOPMENT OF REPTILES. 



619 



(c) The Gavials or Gharials, of the genus Gavialis, are distinguished by 

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

 cits, said to attain a length of 20 ft., 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. 



ALLIGATORS. 



CROCODILES. 



GAVIALS. 



Longer. 



The first bites into a 



The head is short and 

 broad. 



First and fourth lower 

 teeth bite into pits in the pit ; the fourth into a 



groove. 



Not beyond the eighth. 



upper jaw. 



The union of the two 

 rami of the lower jaw does 

 not extend 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 back. 



As in the alligator. 



Unequal. 



Sometimes distinct, 

 sometimes continuous. 



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 bones do not 

 form part of the nasal 

 aperture. 



Almost equal. 



Continuous. 



History of Crocodilians. These giant reptiles form a decadent 

 stock. Fossil forms are found in Triassic strata (e.g. Belodon, Para- 

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

 In Cretaceous strata, crocodilians with procoelous vertebra? 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 segmenta- 

 tion 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 



