288 



THE WORLD OF LIFE 



stages " developed strange knobs and spikes on their shells, so 

 that thej seemed to be trying experiments in excessive vari- 

 ation.'^ 



Figs. 96, 97^ show typical forms of Trilobites (so called 

 from their three-lobed bodies) ; while at a later period, when 

 the whole group was approaching extinction, it produced 

 spined forms like that shown in Fig. 98. 



Excentric forms of Ammonites 



At a later period the wonderfully rich and varied Am- 

 monites show still more curious changes. Beginning in the 



Devonian formation thev increased 

 in varietv of form and structure 

 all through the succeeding forma- 

 tions, till they finally died out in 

 the Cretaceous. The two species 

 here figured from the Trias (Figs. 

 99, 100) may be taken as typical; 

 but the variations in surface pat- 

 tern are almost infinite. Visitors 

 to Weymouth or Lyme Regis maj" 

 find such in abundance under Lias 

 cliffs, or in the former place 

 along the shores of the backwater. 

 As time went on Ammonites in- 

 FiG. ^^ — Acidaspis dufrenoyi. creased in maximum size, till in 



Silurian (Bohemia). 



the Chalk formation specimens 2 

 or 3 feet diameter are not uncommon. One of the largest 

 English specimens in the British Museiim (Xatural History) 

 was found at Rottingdean, near Brighton, and is 3 feet 8 inches 

 across; but the largest known is an allied species from the 

 Upper Chalk of Westphalia, and has the enormous diameter 

 of 6 feet 8 inches. 



It is an interesting fact that the very earliest Ammonites 

 were straight, and gradually became closely coiled. This 

 form was maintained almost constant throu2:hout the vast 



