176 THE ZOOLOGIST. 
and chalk rocks, do not approach within ten miles, but would 
doubtless produce different species of shells. The Ouse Valley 
between these low-lying escarpments is formed of a succession of 
recent deposits lying upon the Keuper and Bunter beds. These 
cannot greatly affect the Conchology of the neighbourhood, as 
they only approach the surface here and there, as near Poppleton 
and Rufforth. In the Ouse Valley itself boulder clays, with 
limestone, sandstone, and igneous blocks, form the chief beds. 
These are separated by glacial sands, which produce numerous 
waste moorlands when forming the surface soil, the clays below 
making them very retentive. Such are Strensall and Riccall 
Commons, Tillmire, and other tracts of equally poor land, though 
now cultivated. The clays often rise to small hills, seventy to a 
hundred feet above the Ouse, such as the site of the Retreat, the 
Mount, Severus’ Mount, &c. Near the river their crests are 
chiefly gravel-beds, evidently the boulder clay rearranged, with 
the lighter parts sifted out. Depressions occasionally contain 
peat, as at Askham Bog, the Foss Islands, &c.; but are generally 
levelled up with brick-earths, which also underlie these peat 
deposits.” 
Our chief localities for water-shells are the broad and sluggish 
Ouse, with its smaller tributary, the Foss, Askham Bog and 
-Hobmoor, the latter being a group of very large and weedy brick- 
ponds, and the former a mixture of brick-ponds, bog, wide marshy 
ditches and wood. For a fuller notice of it and its numerous 
Natural-History productions, including shells, I must refer the 
reader to a series of papers that appeared last year in ‘ The 
Natural History Journal,’ and have now been published in a 
pamphlet form (W. Sessions, York). It appears that there are at 
least forty-one species of Mollusca found within its borders, and 
on a recent visit of the Yorkshire Naturalists’ Union, twenty-three 
were obtained, though Ancylus lacustris and Zonites radiatulus were 
missed. ‘To the localities mentioned must be added numerous 
ponds in the corners of fields, and various ditches and streams. 
Askham Bog and Hobmoor also are among the most productive of 
the localities for terrestrial mollusks, as well as the extensive Jugs 
beside the river at Clifton and Bishopthorpe. In acquiring mate- 
rial for this list I have largely availed myself of the herculean 
labours of our two rivers in the way of shell-collecting. This, 
though a most productive way of obtaining information and 
—— a 
