EVOLUTION, VARIATION AND HEREDITY 477 







to the groups ancestral to all modern Reptilia, except the Chelonia, 

 and to Birds, as well as to the most extraordinary forms of Verte- 

 brates that have lived on the Earth. They all probably originated 

 from a group, the Thecodontia, first known from the Lower Trias. 

 These were comparatively small, little specialised forms, showing 

 certain advances over the Cotylosauria in the structure of the skull 

 and shoulder girdle. From them sprang the Squamata, the Dino- 

 sauria, and, perhaps, also the Rhynchoeephalia. The last group 

 probably appeared in Mid Triassic times, and while resembling true 

 Lizards in external appearance, they nevertheless present important 

 points of difference from them in the characters of their skeleton, 

 notably of the skull. They were apparently never a very extensive 

 group, but one solitary member of it, the New Zealand Tuatara, 

 Sphenodon punclalus, has persisted to the present day. It is now 

 confined to certain islands off the coast of New Zealand, and is in 

 grave danger of extinction at no very distant date. The Squamata 

 also appear in the Mid Tiias, and constitute the true Lizards and 

 other allied forms derived from them. The term Lizard, as applied 

 to living forms, is restricted to the Order Lacertilia, examples of 

 which are first found in late Triassic times. They have persisted 

 until to-day, and now form a fairly widespread group, the Lizards, 

 Iguanas, Monitors, Blind-worms, etc., living in the temperate, sub- 

 tropical, and tropical zones. On the whole, they are adapted for a 

 life on dry land, many of them even to the desert, but some are 

 burrowing forms and a few partly aquatic. From the Squamata, 

 perhaps during the Upper 1 rias, were derived the snakes, or Ophidia, 

 which still persist, although we find no record of them until the 

 Upper Cretaceous. They also gave rise presumably about the same 

 time to the Mosasauria, which were a group of aquatic forms having 

 but a relatively short existence, appearing first in the Upper Cre- 

 taceous and becoming extinct in the same period. In spite of their 

 limited duration they were highly specialised, large in size, varying 

 trom 8-50 feet in length, and were widely scattered over the globe. 

 They left well-preserved fossil remains, including in some cases 

 pieces of skin, and from the fact that they had a powerful set of 

 teeth, and their bones often bear healed scars, we may assume they 

 were fierce fighters. In form they were somewhat eel-shaped, with 

 limbs reduced to small fin-like structures and a powerful tail, which 

 was not of the same fish-tail shape as in the Ichthyosaurs. 



The Dinosauria are a large diverse group that were ap- 

 parently the dominant land-dwelling Vertebrates throughout a 

 large part of the Mesozoic era. They first appear in the Middle 

 Trias, and show two distinct lines of development : the Saurischia, 

 in which the bones of the pelvic girdle resemble those in Lizards, and 



