CAINOZOIC FAUNAS 205 



members of the order to attain importance in the Meso- 

 zoic, are fully represented in the Tertiary. Large 

 species of Arctica are common in the London Clay 

 and Coralline Crag. Astarte is especially abundant 

 in the Pliocene, Crassatellites (PL xv. fig. 7) in the 

 Eocene. Corbicula is common in fresh-water Oligocene 

 deposits, and Cardita (PL xv. fig. 8) abounds in the 

 Bracklesham and Barton series. Chama (especially at 

 the latter horizon) represents the persistent radical 

 from which the Cretaceous Rudistae sprang. Lticina 

 and Corbis are common throughout the era. Cyclodonts 

 are abundantly illustrated by the Cardiidae, Protocardia 

 and Discors are especially frequent in the Eocene, 

 Cardium itself is common in the Crags. Teleodonts 

 are the predominant forms. Venus (PI. xvi. fig. i), 

 Dosinia (PL xvi. fig. 2), Meretrix and Paphia have 

 numerous species in the later Cainozoic; Tellina, 

 Macoma, Psammobia and Donax are no less common 

 as fossils than on modern beaches. Though far from 

 rare in the Eocene, they are particularly common in 

 the Pliocene. Mactra and Spisula are no less abundant. 

 Asthenodonts are more fully specialized and differenti- 

 ated in the Cainozoic than before. The well-known 

 genus Mya is important in the Pliocene, Corbula 

 abounds in the Oligocene, Panope is common in the 

 London Clay, and Teredo (the ship-worm) riddled 

 drift-wood in the Eocene estuary of the London Basin. 



In spite of the abundance of Bivalves, Gastropods 

 are undoubtedly the dominant Molluscan class of the 

 Cainozoic. Aspidobranchs make up in numbers what 

 they lack in variety ; Ctenobranchs show both qualities ; 

 while Pulmonates carry univalve supremacy on to 

 unfamiliar ground. The venerable Patella (a " Lingula " 

 among Gastropods) seems incorrigibly successful. The 

 small " slit-limpets " (e.g. Emarginula) are common 



