EXTENSIONS OF DARWINISM 267 



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. 



FlG. 96. CONOCORYPHE SULTZER1. 

 Upper Cambrian. 



FIG. 97. PARADOXIDBS BOHEMICUS. 

 Upper Cambrian. 



Eccentric forms of Ammonites 



At a later period the won- 

 derfully rich and varied Ammo- 

 nites show still more curious 

 changes. Beginning in the 

 Devonian formation they in- 

 creased in variety of form and 

 structure all through the succeed- 

 ing formations, till they finally 

 died out in the Cretaceous. The 

 two species figured overleaf from 

 the Trias (Figs. 99, 100) may be 

 taken as typical ; but the varia- 

 tions in surface pattern are almost 

 infinite. Visitors to Weymouth 

 or Lyme Regis may find such in 

 abundance under Lias cliffs, or 

 in the former place along the shores of the backwater. 



As time went on Ammonites increased in size, till in 

 the Chalk formation specimens two or three feet diameter are 



. ACIDASPIS DUFRESNOYI. 



Silurian (Bohemia). 



