A Suggestion on Extinction. 475 



now so isolated in the modern fauna, owe their persistence 

 to any retention of more primitive characters than their 

 extinct relations, would be an interesting inquiry. The 

 degree of specialisation and size acquired by the Labyrintho- 

 donts before their extinction in the Trias is only too apparent. 



Amongst the groups of Eeptiles that were so dominant 

 during the Mesozoic we have two persisting, widely dis- 

 tributed, in the Crocodilia and the Chelonia, and one, the 

 Rhynchocephalia, almost on the point of extinction. Of the 

 Chelonia^ Dr A. S. Woodward remarks that " the earliest 

 known remains of the shell from the Upper Trias appear to 

 be as typical as the corresponding parts of a modern tortoise, 

 and the remains of skulls and limbs in the Upper Jurassic 

 are so similar to those of living forms, that they scarcely 

 unite the three suborders as now differentiated in the 

 modern fauna." It is evident, therefore, that this group 

 varies with extreme slowness, and one cannot help uniting 

 with this its persistence in time as well as the sluggishness 

 of the physiological functions and great powers of retention 

 of life exhibited by the order. The great fertility of some 

 of the members of the group is also very remarkable — some 

 turtles are said to lay two hundred and forty eggs or more 

 within a short space of time. 



The Crocodilia have shown increasing specialisation since 

 the early Mesozoic. Their limbs have throughout retained 

 a very generalised condition, and the peculiar specialisation 

 of the fauces, with the position of the eyes and nostrils, was 

 acquired through a long series of forms, and only showed its 

 present degree of perfection in Tertiary times. The croco- 

 diles also are extremely fertile, laying as many as one 

 hundred eggs or more. 



The genus Sphenodon, of New Zealand, now. so nearly on 

 the verge of extinction, is the sole living representative of 

 a group of reptiles which at so early period as the Trias 

 showed a wide distribution in space, and frequently attained 

 large size, with most extraordinary forms of specialisation. 

 Other forms occur through the Mesozoic and also in the 

 early Tertiaries, where Champosaurus appears to have had an 

 aquatic existence. Singularly enough, according to Lyddeker, 



