EVOLUTION IN THE PAST 



POLYZOANS 



BIVALVED 

 MOLLUSCS 



UNIVALVED 

 MOLLUSCS 



South America, but on Australian scenes they made a grand 

 display. The shells were wonderfully varied in shape, pattern, 

 and structural detail ; and there were differences also in the 

 texture and the material of their composition. 



The species with hinged shells had now far outstripped the 

 " unhinged " (Pentamerus). Forms were arising that had 

 developed spines on the shell a new means of clinging to 

 resting-places (Chonetes). The " arms " of some had now 

 become so greatly developed that the shells in order to lodge 

 them had assumed the shape of a butterfly with outstretched 

 wings (Spirifer). And there were many other displays of 

 originality in this plastic group. The animals, in short, were 

 at the height of their glory : but a great sifting-out process 

 was not far distant. 



Whilst the graptolite colonies were declining, conditions 

 seem also to have been unfavourable to the colonies of 

 higher grade, known as polyzoans. The older-fashioned 

 open-mouthed colonies still kept the lead, but they had now 

 lost their great superiority in numbers over the colonies 

 with zooid-tubes of greater complexity. 



Bivalved molluscan life still remained in a primitive ill- 

 defined condition ; but there were signs of progress. Some 

 animals were appearing in a state of development between 

 mussels and pearl-oysters (Myalina). Other forms were more 

 in line with modern wing-shells than were their Ordovician 

 forerunners (Pteria). A few new forms were appearing in 

 exceptionally pretty shells, shaped like a cockle's, and 

 with radiating ribs broken up into small knobs by cross 

 furrows (Cardiola). These creatures seem to have been more 

 nearly related to arks than to cockles : but their family was 

 not destined to a very long career. With better prospects 

 were certain other new forms, in shells of scallop pattern 

 (Aviculopecten). 



Sea-snails did not exhibit much notable development. 

 Members of some families persisting from the Cambrian had 

 their sanitary arrangements in an improved condition. The 

 waste substance had been promiscuously discharged, and must 

 often have polluted the water coming to the gills. It was 

 now carried through a deep notch or slit in the shell, and 



