EXTINCT TYPES. 



189 



equator the series of meridional rows in each intermediate 

 area is increased to four, which continue to the basal pole. 

 Going still further back to the trias, we find another 

 genus of urchins (Tiarechinus) with an increase in the 

 number of rows of plates in the intermediate areas above 

 the lower pole. Moreover, some of the urchins of the 

 Jurassic rocks differ from the ordinary type, in that the 

 plates of the test overlap one another instead of joining by 

 their edges. 



These depar- 

 tures from the 

 normal form in 

 some of the 

 Secondary ur- 

 chins suggest 

 that, if we were 

 to go back to the 

 Palaeozoic epoch, 

 we should find 

 still more marked 

 differences from 

 living types. 

 Such, indeed, is 



actually the case, 

 and we find that 

 existing ones 



in 



Fig. 58. Side view of the Test of a 

 Secondary Sea - Urchin (Tetracidaris). 

 a, ambulacral areas ; * #, intermediate areas. 



all the Palaeozoic urchins differ from 

 the number of meridional rows of 

 plates, while very frequently these plates overlap one 

 another. In the specimen represented in Fig. 59 it will be 

 seen that while the ambulacral areas are normal, the 

 intermediate areas have now no less than five meridional 

 rows of plates ; while in yet another form (Melonites] the 

 ambulacral as well as the intermediate rows are increased 

 in number, the former varying from seven to eight, and 

 the latter from eight to fourteen. With one exception, 

 however, all the Palaeozoic urchins agree with the common 

 species in having the vent situated at the apical, and the 

 mouth at the basal pole, this mouth always having the 

 " Aristotle's lantern." 



Now if we proceed still further back till we reach the 

 lower part of the Palaeozoic epoch, such as the Cambrian 



