BIVALVIA. 139 
regarded as a variety of A. sulcata, the ridged or sulcated form being the British and 
Arctic one, and the smooth form the Mediterranean one. It appears to me, however, 
that where such distinct forms are characteristic of separate areas ‘it is but a question of 
words whether for palzeontological purposes we call them species or varieties, and it is 
significant that in the Cor. Crag, with, as I consider, a fauna having its chief affinities with 
the Mediterranean, we get no trace of su/cata, but have incrassata in profusion. Some 
imperfect specimens have occurred in the Middle Glacial at Hopton that seem to 
belong to this species, but being imperfect I have assigned a Middle Glacial locality to 
it with a note of interrogation. If, however, better specimens should confirm this, we 
should, as the undoubted sw/cafa occurs at Hopton, have both the Arctic and 
Mediterranean forms together in the Middle Glacial deposit. This is probably Zediina 
Jusca, Poli. 
Asrarre suncata, Dacosta. Crag Moll., vol. ii, p. 182, Tab. XVI, fig. 5 a, 6. 
Localities. Red Crag, Sutton. Fluvio-marine Crag, Bramerton and Thorpe ? 
Chillesford bed, Aldeby. Middle Glacial, Hopton. Upper Glacial, Bridlington. Post 
Glacial, March. 
This shell still remains in the Red Crag very rare to my researches. It is given by 
Dr. Woodward in his Norwich Crag list as occurring both at Bramerton and Thorpe, 
the young being more frequent, but I have not been able to confirm this through any 
of my Norwich correspondents. It has been obtained at Aldeby by Messrs. Crowfoot and 
Dowson. In the Middle Glacial at Hopton two or three perfect valves of young 
specimens and one full grown have occurred. It is among the specimens in the British 
Museum from Bridlington, and Mr. Harmer has found it at March. 
Asrarre Omati, La Joné. Crag Moll., vol. ii, p. 180, Tab. XVII, fig. 1 a—/. 
Localities. Cor. Crag passim. Red Crag, Sutton. Fluvio-marine, Bramerton ? 
Middle Glacial, Hopton. 
The principal portion of a shell, as well as some other fragments, which have occurred 
at Hopton, enable me to refer this species to the Middle Glacial sands without 
much hesitation. It is given in Dr. Woodward’s list as from Bramerton, but I have 
not been able to obtain confirmation of this, and have placed a note of interrogation to 
that locality. 
This species is referred by Mr. Jeffreys in his list to 4. undulata, Gould, but its 
identity with that shell I fully considered more than twenty years ago, and the reasons for 
keeping the two species distinct, given by me at p. 180, vol. u, of ‘ Crag Mollusca,’ 
appear to me to be still valid. If we were to strain identities in this way a suite of 
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