24 CIDAHID^. 



The Cidarida are the most ancient type of the Echinoidea. The remains of different 

 forms of this family are found in the Silurian, Devonian, and Carboniferous rocks, as well as 

 in those of the Secondary and Tertiary periods. In his valuable Synopsis, M. Desor describes 

 six genera in this tribe : these are Cidaris, Klein. ; Rabdocidaris, Desor ; Diplocidaris, 

 Desor ; Porocidaris, Desor ; Goniocidaris, Desor ; Palaocidaris, Desor. Of this number 

 three are extinct — Diplocidaris, Porocidaris, and Palceocidaris ; two contain both extinct 

 and living forms — Cidaris and Bab do cidaris ; and one is only found living — Goniocidaris. 



