158 PLIOCENE MOLLUSCA. 



selves afterwards at the Oakley horizon of the Waltoiiian Crag; one specimen 

 only (now in my possession) was found in the upper part of the section at 

 Walton-on-Naze by Prof. Kendall many years ago, and I obtained another at 

 Beaumont, while at Oakley they are not very rare. It is the northern N. cle^pecta 

 and its varieties, however, that occur in these places and in the Red Crag 

 generally, and not the Recent British i\^. iinfiqn(i,-t\\e typical form of which is 

 unknown to me from either Walton or Oakley, its first appearance in the English 

 Crag being at the Newbournian stage, where it is exceedingly rare.^ 



Taken as a whole the dextral shells are comparatively rare at Oakley and in tlie 

 jSTewbournian Crag, while at Butley they are somewhat more numerous, N. despecta 

 being everywhere the predominant form; at the Icenian stage the sinistral forms 

 had well-nigh disappeared. 



The fact that N. confraria appeared first in the Crag basin led AVood to the 

 opinion, opposite to that of Jeffreys, that the progenitor of the two groups may 

 have been left-handed. No specimen, however, is known to me from the Crag 

 Avhich presents the appearance of an ancestral form uniting the two. If the one 

 had been derived from the other in Crag times we ought to find them approaching 

 as we trace their history backwards, but this is not the case. Specimens of 

 N. despecta and its varieties from Oakley differ materially from the sinistral fossils 

 with which they are found, having a shorter spire, a less oblique suture, and 

 different sculpture. 



Several varieties of N. contraria are known from Oakley, but they do not 

 approach those of the despecta group, and no specimen of the former has been found 

 which could be regarded as a reversed, i. e. a right-handed, variety of it. 



I adopt, therefore, the opinion which has been held, from the time of Linne, by 

 foreign conchologists, almost without exception, believing that in Pliocene times 

 these dextral and sinistral shells had become sufficiently differentiated to entitle 

 them to specific rank. Comparing Pis. XVI and XVII, most of the specimens 

 being from the same locality, the difference between the two is rather striking. 



Originating probably in seas to the north of Great Britain, and arriving some- 

 what suddenly in these latitudes, when communication was first opened up between 

 the Crag basin and the north by the tectonic sul)sidence referred to in one of iiiy 

 earlier papers,' it seems clear that at the time of their first recorded appearance 

 they had become separated into distinct groups, which migrated separately, and 

 still survive, nearly in their original form. 



1 There is a specimen in the Jermyn Street Museum of the typical N. antiqna labelled Walton- 

 on-Naze. Its colour, however, is not that of any Walton Neptunea known to me, and Mr. A. Bell, 

 whose knowledge of the subject is unique, expresses a very strong opinion that it is not Waltouian, 

 but came from some Newbourniau horizon. Dr. Kitchin thinks the locality claimed for it may 

 probably be a mistake. 



^ " Pliocene Deposits of Holland," Quart. Journ. G-eol. Soc, vol. lii, p. 754, 1896. 



