62 THE TRIAS. 



cup, a peculiarity regarded by this eminent anatomist as pro- 

 bably fitting the animal for partially marine habits. And that 

 the full importance of this peculiarity of the early reptiles 

 may ' be appreciated, the reader must be made aware that 

 modern reptiles have a ball-and-socket form of the vertebrae 

 that is, a convexity at the one side fitting into the hollow of 

 the adjacent bone ; but this form only when they are mature 

 animals, for in the embryotic state of the crocodile and of the 

 frog the form has been ascertained to be biconcave, which 

 gradually changes as the animal approaches perfection. The 

 teeth of the thecodonts and palseosaurs were fixed in distinct 

 sockets, like those of the modern crocodiles. In this respect, 

 they were superior to the modern varanians, the nearest living 

 tribes, which have the teeth imbedded in comparatively shallow 

 cavities along the bottom of a groove in the jaw. 



ERA OF THE TKIAS AND OOLITE. REPTILES ABUNDANT. 



FIRST TRACES OF BIRDS AND MAMMALIA. 



GEOLOGISTS apply the term Secondary Formation (once of 

 wider application) to those strata intervening between the end 

 of the Permian or close of what they call the Palaeozoic Period, 

 and the termination of the Chalk Series, afterwards to be de- 

 scribed, at which place there is another and almost total change 

 of specific forms. The first of these formations is called with 

 us the Upper New Red Sandstone; it consists in England of 

 only a group of strata of that kind, surmounted by some 

 variegated marls. But on the Continent, below a stratum 

 equivalent to these marls, there is one of limestone, bearing 

 the name of the Musclielkalk, and full of shells. The system is 

 there called Trias, on account of its thus consisting of a triple 

 group of strata. 



