MOLLUSCA. GASTEROPODA 255 



Eocene. Xenophora, Cahjptrcm, Xatica, Mclanatria, Turritella, 

 Cerithium, Rostellaria, Hippochrenes, Rimella, Aporrkais, Cyprcea, 

 Cassis, Cassidaria, Tritonium, Fusus, Clavella, Leiostoma (=Sycum), 

 Pisania, Pyrula, Mvrex, Typhis, Voluta, Yolvtospina, Volutilitkes, 

 Oliva, Aiicitta, Plewrotoma, Conus, Conorbis. 



Oligocene. Xenta, Xeritina, Xatica, Viviparus, Melania, Mela- 

 nopsis, Cerithiv.m, Potamides, Murex, Fusus, Ancilla, Pleurotoma , 

 Limmea, Planorbis, Rissoa, Helix, Amphidromus. 



Pliocene. Emarginula, Fissurella, Trochus, Scala, Turritella, 

 Xatica, Littorina, Capv.lus, Cerithmm, Aporrkais, Trivia, Biiccinum, 

 Liomesus {=Bmcinopsis), Tritonofusa.s, Chrysodomus, Xassa, Pur- 

 pura, Trophon, Scaphella, Actreon. 



Hyolithes, Conularia, etc. 



Hyolithes, Conularia, Tentaculites and other allied 

 genera, which are often abundant in the Palaeozoic and 

 are almost confined to the deposits of that period, have 

 been considered by many authors to be Pteropods. Their 

 shells, however, are much larger and thicker than those 

 of living Pteropods, and were in some cases {Conularia) 

 attached by the apical end. Since Pteropods are now 

 believed to be highly specialised forms of the Opistho- 

 branchia, it is unlikely that they would be represented 

 in the Lower Palaeozoic deposits. Some writers have 

 suggested that these Palaeozoic genera are allied to 

 primitive forms of Cephalopods (e.g. Volborthella) ; this 

 view receives some support from the presence of septa 

 in the shell, and from the occurrence in Hyolithes of a 

 siphonal lobe (p. 267) suggestive of the existence of a 

 siphuncle. For the present, however, the systematic 

 position of these genera must be regarded as doubtful. 



