62 PLIOCENE MOLLUSCA. 



Fossil: Coralline Crag: Gedgrave (very rare), Bojtoii. Eed 

 Crag : passim. Icenian : Thorpe near Norwich, Postwick, Easton Bavent, 

 Southwolcl. Wexford gravels; Isle of Man; Slains, Aberdeenshire. 



Belgium : Diestien ; Scaldisien ; Poederlien. Holland : Diestien ; Scaldisien ; 

 Amstelien. 



Remarks. — There is no consensus of opinion among palseontologists as to the 

 amount of departure from a given type which justifies the initiation of a new species. 

 In the present case a number of shells, differing essentially in form and sculpture, 

 which in a collection of some hundreds of specimens from one spot (as from 

 Oakley) may be generally separated without dijEficulty, are regarded as varieties of 

 one species. Italian geologists, on the contrary, have described many nearly allied 

 forms of Nassa as separate species, as will appear in the sequel. Had Bellardi, for 

 example, been the first to study the Crag Nassas he probably would have regarded 

 some of our varieties of N. reticosa as specifically distinct.^ 



N. reticosa appears somewhat suddenly in the Anglo-Belgian basin, occurring 

 in great abundance in the Red Crag, but it has now disappeared. The different 

 varieties of this group may perhaps be regarded as incipient species, affording 

 an illustration of the way in which species may have arisen, and explaining the 

 fact that in many cases where specific separation has been apparently established, 

 allied forms show so often a tendency to run into one another. 



N. reticosa has been recorded very rarely from the Gedgravian or older 

 portion of the Coralline Crag, but has been found in some abundance in the latter 

 deposits of Boyton and Ramsholt. Specimens from those places are less worn chan 

 those of the Butleyan zone of the Red Crag, to which, for other reasons, I believe 

 the undoubted Red Crag beds of the Boyton district should be referred. 



As stated in the introduction (p. 4), Mr. Alfred Bell considers that the 

 Coralline Crag fauna of Boyton has certain features which separate it from that of 

 any recognised horizon of the Crag deposits. He suggests it may be intermediate 

 between the Gedgravian and the Waltonian, probably as an upper zone of the 

 former.^ He finds an assemblage of shells in the upper part of the Coralline Crag 

 at Ramsholt, similar to that of Boyton. 



I have little hesitation in accepting the view that the specimens of N. reticosa 

 found at those places are of Upper Coralline Crag age, and that this form should 

 be included in the fauna of that horizon. 



M. van den Broeck, moreover, reports this shell from the Diestien deposits 

 {zone a Terehrafula grandis) of Belgium. 



The fossils figured by Prof. Kendall from the Isle of Man as N. serrata a])pear 



to belong to the present species; they are not the Italian N. serrata of Brocchi. 



« - — 



^ Some specimens of N. auhicjniensis from the Lower Pliocene of Bosq d'Aubigny which I have 

 received from M. Dautzenberg, approach very nearly the Crag var. elongata. 

 ' Jouru. Ipswich Field Club, vol. iii, p. 5, 1911. 



