682 PLIOCENE MOLLUSCA. 



Specific Characters. — Shell smooth, globose, moderately solid ; whorls 6, tumid, 

 slightly flattened below the suture, rapidly enlarging, the last expanded, much the 

 largest; spire short, slightly projecting; mouth large, semilunar, angulate above, 

 expanded below ; suture well-marked, nearly straight ; outer lip incurved upon 

 the body-whorl; inner lip broad upwards, forming a ridge separating the 

 umbilicus from the mouth ; umbilicus of moderate size, deep, open ; operculum 

 horny, not calcareous. In the recent shell the upper part of the whorls is orna- 

 mented by a spiral band of prominent and oblique longitudinal streaks which, 

 however, have been obliterated on fossil specimens. 



Dimensions. — (Of Crag specimens) L. 26 — 30 mm. B. 24 — 28 mm. 



Distribution. — Recent: British seas (sublittoral zone), common. West Euro- 

 pean from Norway to Gibraltar, Mediterranean, widely diffused. 



Fossil : Waltonian Crag : Walton-on-Naze, Beaumont, Little 

 Oakley (rare). Newbournian, Butleyan, Icenian. Weybournian : Belaugh. 

 Middle Glacial Sands: Billockby, Hopton. Manx beds. Wexford gravels. 

 Generally present in the Pleistocene deposits of England, Ireland and Scotland. 



Scaldisien, Poederlien : Belgium. Scaldisien, Amstelien : Holland. 



Upper Pliocene : Altavilla, Pleistocene : Monte Pellegrino, Ficarazzi, Nizzetti 

 near Catania (Scalia). 



Bemarks. — The typical form of this well-known and common British species 

 seems to have made a comparatively late appearance in the Crag basin. It was 

 unknown to Wood from the Coralline horizon, and has been reported but rarely 

 from the Waltonian, being found, however, more commonly in the upper portion 

 of the Red Crag, as, for example, at Butley, as well as in the Icenian and in the 

 British Pleistocene. 



By some authorities N. catena has been identified with the .V. helicina of 

 Brocchi, by others the difference between the two has been regarded as varietal. 

 The typical A. helicina, however, is an extinct form, characteristic of the Miocene 

 and Lower Pliocene deposits, A. catena being a species still living and, as just 

 stated, characteristic rather of the latest horizons of the Crasr and of the English 

 Pleistocene. These shells, which also differed from A. helicina in Crag times, can 

 be, I think, easily distinguished. Believing, as I do, that they have an important 

 zonal value, it seems to me desirable to treat them as specifically distinct. 



Wood's figure of A. catena (op. cit., pi. xvi, fig. 8) is typical, as is my fig. 1 

 of PI. LIV ; they have an expanded and flattened body-whorl. 



MM. Dollfus and Dautzenberg agree with me that .V. catena, Linn., and 

 A. helicina, Sacco (Brocchi), should be considered different species. 1 



1 The subgeneric name Lunatia, Gray (1847), has been adopted by Scandinavian authorities in 

 place of Naticina (1834). The latter, however, being the older^has been, until lately, more generally 

 preferred. M. Dautzenberg, however, has informed me that he now considers his reference of the 

 present species in 1883 and in 1912 to Nntirlna was a mistake. 



