490 



THE IRREGULAR SEA-URCHINS. 



Pseudopedina, Stomechinus, Polycyphus, of which only Stomechinus 

 survives to the Neocomian, in which the living genus Psammecliinus 

 is found, together with Pedinopsis, Gflyptechinus, and Codechinus. 

 The cretaceous types are Microped.ma and Diplotagina. The Lower 

 Tertiary genera are Echinopedina and Leiopedina, while Echinus and 

 many other surviving genera date from this period. 



The irregular sea-urchins called Exocylica, in which the two poles 

 are not opposite, have a similar range in time. Among them Pygaster 

 commences in the Lias, and ranges to existing seas, accompanied by 

 Holectypus as far as the Neocomian. Pileus characterises the Upper 

 Oolites, Discoidea ranges from the Neocomian to the Chalk, Galerites 

 is typically Cretaceous. Conoclypus commences in the Chalk, and 

 lias one existing representative. 



The Euclypeasters commence in the Cretaceous beds with Eclii- 

 nocyamus, which characterises the Tertiary, and is still living. It is 

 associated with Fibularia. Lower Tertiary genera include Sismon- 

 dia, Scutellina, and Lenita. Clypeaster and Lagamum first appear 

 in the Lower Tertiary. 



The Scutelline urchins commence with the Tertiary, and are 

 represented by the extinct genus Mortonia, and by species of Scutella, 

 Amphiope. We then pass to the urchins in which an elongated form 

 replaces the hitherto circular base, and in which both poles depart more 

 or less from the central position. The Cassidulidce present two types 

 first, that of Galeropygus, which appears in the Lias, and ranges 

 through the Oolites, in which it is accompanied by Hyboclypus, 

 Galerodypus, and Infradypeus. Pyrina commences in the Lower 

 Oolite, and survives to the Lower Tertiary. 

 Echinoneus is found in the Middle Tertiary, 

 and still exists. The closely allied group of 

 Ecliinolampidce commences with Clypeus in the 

 Lower Oolite, and in the Middle Oolite are 

 found Nucleolites and Pygurus, which range up 

 to the Neocomian, unless the living genus 

 Fig 103. Nucleolites Nucleolites prolong it to existing seas. . Cato- 

 pygus is a Neocomian type, which is still 



living. Cassidulus commences in the Cretaceous, and is found in the 

 Tertiary. Other Tertiary genera are Echinolampas and Rhynchopyyun, 

 which are still living. Collyrites appears 

 with the Lower Oolite, and survives to the 

 Xeocomian. The Ananchytidce begin in the 

 Neocomian with Holaster, which survives 

 to the Middle Tertiary. Other Cretaceous 

 genera are Cardiaster, Infidaster, and Hemi}h 

 neustes. Anancliytes survives from the Chalk 

 to the Middle Tertiary. The Spadangida; 

 are represented in the Neocomian by Heter- 

 aster, Toxaster, and Enalaster. The Creta- 

 ceous rocks yield Hemiaster, Epiaster, Micraster, and Prenaster. The 

 Lower Tertiary are characterised by Macropmustes, Peripneustes, 



Fig. 104. Auanchytes. 



