CAINOZOIC FAUNAS 201 



group. Echinus, and a number of allied genera, have, 

 on the contrary, reached a sufficient acme at present. 

 E. esculentus abounds around rocky districts of modern 

 shores, and a closely similar species occurs, usually in 

 a much shattered condition, in the East Anglian 

 Pliocene. The predominant Cainozoic group of Regular 

 Echinoids is the Echinometridae. Parasalenia> which 

 arose in the Eocene, is not much advanced beyond the 

 EMnus-stage of elaboration, but Strongylocentrotus 

 (which is locally abundant on British rocky fore- 

 shores), Echinometra and Heterocentrotus (tropical genera) 

 show the highest degree of specialization known in the 

 subclass. 



Among Irregular Echinoids, the Holectypoida have 

 dwindled to the solitary genus Echinoneus, which is 

 found fossil in the Mediterranean region, but lives on 

 the other side of the Atlantic at present. However, 

 direct descendants of the Holectypoid Discoidiidae, the 

 Clypeastroida, have shown an extraordinary evolutional 

 activity throughout the era. Small, retarded forms, 

 such as EchinocyamuS) occur in the British Pliocene and 

 live around our shores to-day, but the main stream of 

 Clypeastroid progress has flowed in subtropical climates. 

 In the Eocene, Miocene and Oligocene rocks of the 

 Mediterranean area, small discoid forms like Sismondia 

 and Scutellina are overshadowed by the extreme variety 

 and numbers of large Clypeastridae and Scutellidae. 

 Clypeaster (PL xv. fig. 3), Scutella and Encope are perhaps 

 the chief genera, and from the Himalayas to the Andes 

 are local rock-formers at every Cainozoic stage. 

 Nucleolitoids are only not extinct, but their place is 

 taken by the true Cassiduloida, which, in such genera 

 as EchinolampaSy vie with the Clypeastroids for 

 supremacy. Heart-Urchins are hardly less abundant, 

 but the delicacy of their tests makes them less con- 



