PROSOBRANCHIATA. 



779 



675. — Natica clausa. 

 Post-Pliocene. 



spiral, with an excentric nucleus, and may be either horny or 

 calcareous. 



The shell in Natica (fig. 675) is globular, with a large body- 

 whorl, usually smooth and polished, and often with coloured mark- 

 ings. The inner lip is callous, and the shell 

 is umbilicated, though the umbilicus may be 

 partially or wholly filled up. A large number 

 of recent, and a still larger number of fossil, 

 forms of Natica are known, species having 

 been described from rocks as old as the Silu- 

 rian. It is, however, very questionable if any 

 true forms of Natica are known from any 

 Palaeozoic deposit, anterior to the Carbonifer- 

 ous at any rate. The most typical forms of 

 Natica, in a strict sense, possess a wide umbil- 

 icus and a twisted columella, and the oldest shells of this type are 

 from the Eocene Tertiary. 



Ampidlina includes Eocene and Miocene forms, in which the columella 

 is only slightly thickened and is not twisted. Amauropsis includes a 

 number of fossil forms, of which the earliest perhaps appear in the Car- 

 boniferous Limestone, while many others are found in the Secondary 

 rocks and the Eocene. In these types there is no umbilicus or only a 

 narrow umbilical fissure, and the spire is moderately high. Lunatia, 

 again, includes a series of forms, ranging from the Trias to the present 

 day, in which the aperture is semilunar, and the umbilicus is of moderate 

 or small size, and is without a spirally-twisted callus. The Eocene and 

 Oligocene genus Deshayesia has the inner lip covered with a thick callo- 

 sity, which largely conceals the umbilicus and is toothed on its apertural 

 edge. The Jurassic and Cretaceous genus Tylostoma, lastly, includes 

 forms with an elevated spire, and without an umbilicus, the outer lip 

 being thin and sharp. 



The genus Narica ( Vanikord) includes forms in which the shell 

 has a general likeness to that of Natica (fig. 676), but the surface 

 is spirally striated, or 

 cancellated. The aper- 

 ture is semilunate or 

 oval, and the columella 

 is excavated or umbili- 

 cated. The genus ranges 

 from the Jurassic period 

 to the present day. Nati- 

 cella (Silurian to Trias) 

 resembles Narica, but 

 has no umbilicus or a 



small one. Lastly, in Sigaretus (fig. 677) the shell is ear-shaped, 

 with a spirally-striated surface, a minute spire, and a very large 



Fig. 676. — Xarica (Van- 

 ikord) Genevensis, from the 

 Cretaceous rocks. 



Fig. 677. — Sigaretus 

 clathratus, from the 

 Eocene. 



