152 SUPPLEMENT TO THE CRAG MOLLUSCA. 
Rose’s collection from the Nar Valley Brickearth, and it is mentioned by Mr. Seeley 
(under the name of 7. proaima) as occurring in the gravel of March, though Mr. Harmer, 
who has searched the gravel of that place very assiduously, has not been able to meet 
with it. 
The shell which I have called /ata was figured by Lister, and called by him Zel/ina 
lata alba, and the name /ata was adopted by Gmelin. This is the oldest notice of the 
shell. The late Edward Forbes, ‘Mem. Geol. Sur.,’ 1846, p. 412, considered 7. obliqua 
as merely a variety of Lister’s species from the seas of Norway. Although I have the 
highest respect for the opinion of the late E. Forbes (whose premature loss all geologists 
deplore) I must in this instance dissent from it, notwithstanding that it has been adopted 
by Mr. Jeffreys. Similarly E. Forbes regarded 7. pretenuis as merely a variety of this 
shell, and Mr. Jeffreys has followed him. The word variety as distinguished from species 
is, to my apprehension, too vague to carry any precise meaning with it, but as regards 
these three forms, /ata, obliqua, and pretenuis, there can be no question as to their 
complete distinction. 7. obdliqua is the only one of the three forms found in 
the Cor. Crag, but in the Red Crag lymg between the Stour and the Alde we get this 
shell and 7 pretenuis abounding together, and 1’. data very rare, while in the Fluvio- 
marine Crag and Chillesford bed we get all three forms in abundance and well marked. 
I am sure that all collectors from the Crag will bear me out in saying that the three 
forms thus occurring in profusion together can be without difficulty selected, and that 
they do not form merely the terms of an undistinguishable series. They were therefore 
three distinct forms hving in the same sea, and not intermingling; and what else const- 
tutes species ? ‘Two of them are, so far as I am aware, not known living, for nothing 
recent that I have seen can justly be identified with either oddiqua or pretenwis. In the 
first formation upwards from the Crag, viz. the Lower Glacial sands, 7. pretenuis becomes 
rare, while od/:qua maintains itself there in great profusion. As we ascend in the order 
of formations we lose the form pretenuis altogether, but still meet with od/qua in the 
Middle Glacial sands and at Bridlington, but it has not yet been met with in any Post- 
glacial bed, or in any of the English Newer Glacial, or in any of the Scotch beds ; while 
lata not only occurs in these beds, but survives as an Arctic shell. The history of these 
most important shells (for it is the most abundant species that are the most important in 
a geological point of view) is thus clearly traceable in time through the various formations 
much in the same way as I have traced that of the common Zrophon antiquus. 
Asra ALBA, VW. Wood. Crag Moll., vol. ii, p. 237, Tab. XXII, fig. 10. 
Localities. Cor. Crag, Sutton. Red Crag, Walton, Sutton, Bawdsey. Fluvio-marine 
Crag, Thorpe, by Norwich? Bulchamp? Chillesford bed, Chillesford? Aldeby? Post- 
glacial, Nar Brickearth, Pentney (£ose). 
