6 PLIOCENE MOLLUSCA. 



term originally suggested by the late Dr. S. P. "Woodward. I shall allude to them 

 in the following pages as the Norwich, Chillesford, and Weybourne zones of the 

 Icenian Crag. 



At the conclusion of this Memoir I propose to offer a few remarks as to the 

 conditions under which the different Crag beds may have originated, their relation 

 to each other and to other Pliocene deposits, and generally as to the conclusions 

 which a prolonged investigation of the subject may have suggested. 



I offer no apology for dealing at some length with the Mollusca, both recent 

 and fossil, of other regions than our own, the study of which, indeed, has thrown 

 much light on Crag problems. 



During repeated journeys to the south of Europe I was able, not only to visit 

 many Museums in France and Italy, but to collect largely on my own account 

 from the fossiliferous deposits of those countries. I have been long familiar with 

 the Belgian Crag, while as to that of Iceland I have had the privilege of examining, 

 with Dr. Ravn, Morch's type-specimens in the Geological Museum at Copenhagen, 

 and of figuring those which seemed to correspond with undescribed forms from our 

 own deposits. As to the Manx fossils, Mr. A. Bell has been kind enough to make 

 a special visit to the Isle of Man, in order to study the collections of Mr. Kermode 

 and the Rev. S. N. Harrison, and has selected certain examples which, with some 

 others from Jermyn St., I am permitted to figure and describe. 



The importance of the careful study of varietal forms is now universally 

 recognised. In some Crag species so-called varieties differ more widely than shells 

 which in other groups are considered worthy of specific rank. In certain cases, 

 moreover, A'arieties have a distinct zonal value; Purpura lajnllus, for example, is said 

 to occur at all horizons of the Red Crag, but the prevalent varieties of this species 

 in the earliest or Waltonian stage are different to those characteristic of the Butleyan 

 or the Icenian. Indeed the typical P. lapillus of our British seas is not known from 

 the former and occurs but sparingly at the latter. In a recent work Prof. Sollas 

 has urged the importance of a " minute attention to details, not omitting the most 

 insignificant "; his words are specially to be commended to students of the Crag 

 where some interesting problems are still waiting solution. 



I liave not attempted to give a complete bibliography of the various species 

 here dealt with, but have referred only to those Avorks which I think may be the 

 most useful. 



As the present Memoir is intended to be supplementary to Wood's Monograph I 

 have followed, more or less nearly, and for the convenience of students, the 

 arrangement of the Marine Mollusca adopted by him, and as to the non-marine 

 forms, that of the Conchological Society of Great Britain. 



Cringleford, near Norwich ; P. W. HARMER. 



April, 2W,, 1913. 



