]7() PLIOCENE MOLLUSCA. 



task, nor is it lessened when we include that of those still existing in northern 

 seas. Much information on the subject has been published during recent years, 

 however, and specimens for comparison are more easily obtained than formerly. 

 The large number of such fossils I have collected at Little Oakley makes the 

 investigation of the subject not only specially interesting, but very perplexing, one 

 difficulty being that even where these shells may be referred to existing species, 

 they are not always identical with them. This, however, is not surprising; the 

 Pliocene mollusca are separated from those of the present day ])y a period which, 

 though geologically unimportant, is, when measured in years, of great length, and 

 much variation may have taken place in the interval. Unfortunately, many of the 

 Crag mollusca, where they differ from the Recent shells to which they are affiliated, 

 have to be regarded as varieties of the latter, Avhich is like " putting the cart 

 before the horse." Obviously variation must have been a departure, not from the 

 newer, but from an older type. 



It seems probable that among the Pliocene Siphos of the Anglo-Belgian basin 

 there are a number of incipient species whicli had not long diverged from some 

 common ancestral type.^ A portion of these survive unaltered in British or 

 Scandinavian seas or in the depths of the ocean, while others have altogether 

 disappeared. 



In 1848, Wood described some fossils (Mon. Crag Moll., pt. i, p. 40, tab. vi, 

 fig. 10) under the name of Trophon gracile. In the British Conchology (vol. iv, 

 p. 336) Jeffreys expressed the opinion that none of them were identical with that 

 species, considering they agreed rather with a North American form, whicli he 

 said was smaller and more tumid, with a shorter spire, suggesting for the Crag 

 fossils the name of Fusus eurtus. The latter species was never described or 

 figured, but as Jeffreys stated in his list, published in Prestwich's well-known 

 paper of 1871, that the shells called by Wood T. gracile occurred in the Red Crag 

 "passim,'' there can be little doubt it included the group so common in those 

 deposits which I have described in the sequel under that somewhat inappropriate 

 name.^ 



In 1879, in his 2nd Supplement (p. 7), Wood agreed that the shells figured in 

 1848 were not S. gracilis, referring two of them (figs. 10 a and 10 c) to Fitsus 

 (Siplio) Olavii, Morch, and a third (10/>) to S. fortuosus. Reeve. S. Olavii was 

 insufficiently described by Morch in the Geological Magazine for 1871 (vol. viii, 

 p. 396), but not figured. It appears, however, to be a different shell (see p. 193). 



1 Mr. Bell informs me there are boxstones (older Pliocene) in the Ipswich Museum and else- 

 where containing several distinct forms of Sijjlio (see also Journ. Ipswich Field Club, vol. iii, p. 8, 1911) . 



2 Verlcriizen supposes that Jeffreys' 8. eurtus was Fusus striatus, Reeve (Jahrb. Deutsch. Malak. 

 Gesellsch., 1881, p. 8). The use of the word "passim," however, is antagonistic to that view, as I 

 know of nothing occun-ing abundantly in the Crag that can be referred to the latter species. S. striatus 

 is, moreover, a very different shell. 



