206 INVERTEBRATE PALAEONTOLOGY 



Pliocene fossils ; but Pleurotomaria can show only four 

 living species (and few post-Cretaceous ones) of great 

 rarity. The small Trochidae and Neritidae are the most 

 successful series of recent Aspidobranchs. Trochus is 

 abundant in the Pliocene ; Nerita (PI. xvi. fig. 3) and 

 Neritina swarm in the Oligocene, their shells often 

 retaining traces of coloration. Among Ctenobranchiata, 

 Gymnoglossa are represented by small forms akin to 

 Turbonilla, but they are never common. Scalaria is 

 an abundant genus of Ptenoglossa. Taenioglossa are 

 very fully developed. Littorina, which congregates 

 in swarms on modern coasts, is hardly less abundant 

 in the Pliocene. Capulus emulates Patella in persistence 

 as in form ; Calyptraea and Xenophora are essentially 

 Cainozoic genera. Naticidae with greatly inflated 

 body- whorls (Ampullina, PI. xvi. fig. 4) are among the 

 commonest fossils of the Eocene, and abound in the 

 Crag. Fresh-water Viviparidae, strikingly like Vivipara 

 of modern rivers, are common in the Oligocene. Tur- 

 ritella (PI. xvi. fig. 5) is at its best in the Eocene, some 

 species attaining very great size. Melania, Cerithium 

 (PL xvi. fig. 6) and Potamides are common throughout 

 the Tertiary, the two first occurring in marine deposits, 

 the last in brackish layers of the Oligocene. Aporrhais 

 is a well-known Pliocene fossil ; while the small Rimella 

 and gigantic Hippochrenes abound in the Eocene. 

 Cowries are especially Cainozoic forms ; Trivia occurs 

 profusely in the Pliocene and Pleistocene. Large 

 species of Pyrula are common in the London Clay. 



Rachiglossa and Toxoglossa are pre - eminently 

 Tertiary groups. The former are represented by 

 abundant Buccinidae (" whelks ") throughout the era, 

 Buccinum, Pisania^ Nassa, Strepsidura^ Melongena and 

 Neptunea being the most important genera. The last- 

 named is particularly interesting, in that its most 



