78 ANIMAL LIFE PAST AND PRESENT. 



before proceeding to discuss the structure of these 

 creatures, a word is advisable as to the geological 

 position of this formation. The Lias comprises a thick 

 series of beds, usually of a bluish colour, lying very low 

 down in that enormous series of rocks known as the 

 Secondary system, of which the Chalk is the topmost 

 member, while the middle part of the system is formed 

 by the great series of Oolites and their accompanying 

 clays. The Secondary system itself, it need hardly be 

 said, lies upon the upper part of the Primary system, 

 as represented by the Coal-Measures ; while it is suc- 

 ceeded by the overlying deposits of the Tertiary system. 

 In regard to the upper parts of the latter system, it 

 may be possible to make some more or less vague ap- 

 proximation in terms of thousands of years as to their 

 real age, but in the Secondary period any such approxi- 

 mation is totally impossible, and we can only reckon the 

 age of the various beds by geological periods, as repre- 

 sented by the vertical thickness of overlying rock. 

 Now, since the Chalk may exceed 1000 feet in thick- 

 ness, and there are several deposits of equal bulk be- 

 tween this and the Lias, some faint idea may be thereby 

 conveyed of the vast lapse of time by which we are 

 separated from that period when the mud of the Lias 

 was deposited in the shallow seas of what is now Eng- 

 land. The Fish-Lizards were, however, by no means 

 confined to this remote Liassic period, since they 

 also occur not only in the underlying New Red Sand- 

 stone, or Triassic series, which forms the very base of 

 the Secondary system, but likewise lived on through 



