58 THE LESSON OF EVOLUTION 



The hingeless Brachiopods were the first of their 

 class to appear. According to Professor C. E. Beecher, 

 Paterina of the Lower Cambrian approaches nearest to 

 the primitive stock, for it closely resembles the embry- 

 onic shell of later forms ; and during the Cambrian 

 period it gave rise to at least five different types of 

 Inarticulata, one of which represented probably by 

 Kutorgina led up to the Articulata, the first known 

 of which, Orthis, had become well established in the 

 Upper Cambrian. 



Trilobites form by far the most important part of 

 the fossil fauna, and can generally be distinguished by 

 the shortness of the pygidium. Some were as much 

 as eighteen inches in length. The minute Protocaris 

 of the Lower Cambrian is a Phyllopod with a sub- 

 quadrate carapace. 



The Cambrian mollusca are remarkable for the 

 proportionately large number of elongated shells 

 formerly classed as Pteropoda. Lately, however, many 

 biologists have been led to the conclusion that the 

 Pteropoda have had a comparatively late origin. It 

 is now generally allowed that the Hyoliihidce, which 

 includes all the pre-Cambrian and Cambrian forms, 

 do not belong to the Pteropoda. They must, however, 

 be considered as pelagic mollusca, and probably as the 

 ancestors of the Cephalopoda, True Cephalopoda 

 appeared in the upper Cambrian but did not attain 

 any importance. Undoubted Gastropoda are repre- 

 sented by conical and spiral shells, all of which are 

 very thin, and probably belonged to pelagic animals. 

 The spiral shell is strongly in favour of these early 

 Gastropoda having been proso-branchiate ; and this 

 agrees with the discovery that some of the Opistho- 



