592 REPTILES. 



In Birds also small villi of the yolk sac absorb yolk, and others on the 

 allantois absorb albumen. (See A. C. Haddon's Embryology.) 



Extinct Reptiles. 



The first known occurrence of fossil Reptiles is in 

 Permian strata ; in the Trias most of the orders or classes 

 are represented; while the "golden age" of the group was 

 undoubtedly during Jurassic and Cretaceous times. 



Some of the modern Reptiles are linked by a series of 

 fine gradations to very ancient progenitors, the Crocodiles 

 of to-day lead back to those of the Trias, the New Zealand 

 Hatteria to the Triassic Rhynchocephalia, but we have no 

 example of a Reptilian genus which has persisted from age 

 to age as Ceratodus has done among fishes. It follows 

 naturally from this finking of the present with the past, that 

 among the fossil forms we find " generalised " types, types 

 which exhibit afifinities with groups, which in our classifica- 

 tion of recent forms may be very widely separated. It is 

 indeed, as has been said, only because of our ignorance of 

 their past history that we are able to classify living genera 

 into separate orders at all. 



The following types of extinct reptiles seem to have en- 

 tirely disappeared : — 



Aiwniodontia. — Lizard-like animals with limbs adapted for walking, 

 found in the Permian and Trias. The nrost generalised forms approach 

 the Labyrinthodont Amphibians very closely, especiall}' in the characters 

 of the skull and pelvis. They, however, also exhibit affinities with the 

 Monotreme Mammals. In the more specialised types the nature of the 

 dentition is the most interesting feature. In GaU'saicnis^ for example, 

 which is a carnivorous form, the teeth are arranged in three series, the 

 anterior series (incisors) are separated by a tusk-like tooth (canine) from 

 a lateral series of cheek teeth (molars). It is hardly necessary to insist 

 upon the close affinity between such a dentition and that of carnivorous 

 ISIarsupials, and we cannot doubt that the Anomodontia are in some way 

 related to Mammals. 



Examples : — Panasaunts, Galisaiinis, Dicynodon. 



Saurop/erygia. — Reptiles represented from the Trias to the Chalk, 

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

 limbs vary ; in the earlier, more generalised, forms they are adapted for 

 walking on land, but in the more specialised types they are modified 

 into powerful paddles, like those of Chelonia. The nearest affinities are 

 with the Chelonia. Notosaiinis had limbs adapted for progression on 

 land ; Plesiosaunis and Plwsaiinis were carnivorous forms adapted to 

 an aquatic life. Plesiosaunis had a veiy long neck, and sometimes 

 attained a length of 40 feet. In Pliosaiiriis the neck was much shorter. 



