PURPURA DBRIVATA. 119 



the group characteristic of the Waltonian deposits had well-iiigli disappeared from 

 the Crag sea before the arrival of the existing forms, suggests that the former may 

 have been due to an earlier migration, possibly more or less contemporaneous with 

 that which ])rought Neptunea (■oiitraria and some other Waltonian species into the 

 Anglo-Belgian basin. The different Crag varieties are evidently closely allied, but 

 at present we have no evidence as to the ancestral form from which they may 

 have sprung. It seems, however, that they liave a stratigraphical value, indicating 

 the relative age of the deposits in which they occur. 



Edward Forbes expressed the opinion that the British ]\ lapillus is of 

 American origin, having been introduced to European seas during the Pleistocene 

 epoch. That the Purpuras which now range from Scandinavia to Behring Strait 

 had some common circumpolar origin seems probable ; there is no evidence, 

 however, to show Avhether they travelled to the east or the west of their original 

 home, or in both directions at once, but as these shallow-water forms had arrived 

 in these regions during the Crag period, their migrations must have been, I think, 

 pre-Pleistocene, and have taken place at a time when the noAv sul:)merg'ed 

 (Ireenlando-European ridge was in existence, and the northern coasts of North 

 America were less encumbered with ice than they are at present. 



Jeffreys says that P. lapillus is found on the coast of Brittany, in Vigo Bay, 

 Senegal, Teneriffe, the Azores and elsewhere, but what relation these southern 

 shells may bear to the forms now existing in the North Sea I do not know. The 

 species in question is not reported, either Recent or fossil, from the Mediterranean ; 

 there and elsewhere it seems to be represented by an allied form, P. haemastoiaa, 

 which also occurs fossil in the Pleistocene of Sicily. An imbricated variety of 

 the recent P. lapillus is still found in British seas. It is the habit of imbrication, 

 however, rather than any special form which survives. The imbricated shell of 

 Oakley belongs, as we have seen, to the older and carinated group ; a Recent 

 specimen in my collection from Copenhagen and that figured by Prof. Sars, on the 

 contrary, are merely imbricated replicas of the existing form. 



Purpura derivata, sp. nov. Plate XII, fig. 30. 



1837. Cf. Purpura exilis, Partscb, v. Hauer, Neues Jahrb. f. Min., p. 417, uo. 42. 

 1856. Cf. Ptirpura exilis, Homes, Foss. Moll. Tert. Wien, vol. i, p. 169, pi. xiii, fig. 21. 



Dimensions. — L. 35 mm. B. 30 mm. 

 Distribution. — Not known living. 



Fossil : Waltonian Crag : Little Oakley (derivative). 

 Remarks. — The worn specimen from Oakley here represented, doubtless deriva- 

 tive from some older deposit, approaches that given by Homes as P. exilis, a 



