ORIGIN AND RADIATION 5 



on each side in the form of valves which can be drawn together 

 by a muscle running across the body. Many fossils are known which 

 look like Conchostraca and date from the Cambrian period (about 

 400 million years ago), but it is uncertain how many of these are 

 true conchostracans because nothing is known about the limbs. 

 However, it seems fairly certain that they were in existence at 

 about the same time as Lepidocaris. The Cladocera (fig. 4) are 

 smaller than the Conchostraca and have fewer limbs; the carapace 

 is similar in form although it does not extend forwards to enclose 



EXCRETORY 

 ORGAN 



PROLONGATIONS 

 FROM, 1ST LEG 



Fig. 5. Triops granarius (Notostraca) dorsal view. Speci- 

 men from South Africa, actual length 2.5 cm. excluding 

 the caudal furca. The European species, T. cancriformis, 

 has a relatively shorter body than this species. 



