"Parr V. Szcr. i. § 2.) PLEISTOCENE. 895 
fragment torn from a submarine shell-clay, and imbedded in the 
boulder-clay. With the exception of such marine enclosures the organic 
contents as well as the physical characters of the Scottish till point to 
terrestrial conditions of deposit under the ice-sheet. 
_ The records of the submersion of Britain are probably very incom- 
plete. If we rely only on the evidence of actual marine shells we obtain 
the lowest limit of depression. But the renewed ice and snow, after 
re-elevation, may well have destroyed most of the shell-beds, and their 
destruction would be most complete where the snowfields and glaciers 
were most extensive. Beds of sand and gravel with recent shells have 
been observed on Moel Tryfaen, in North Wales, at a height of no less 
than 1350 feet, but as the same kind of deposits in which they occur 
extend to a much greater height, the submergence may have considerably 
exceeded the limit at which the shells occur. In Cheshire beds of shells 
have been met with at a height of 1200 feet. In Scotland the highest 

Fic. 426. Group oF SHELLS FROM THE ScoTTIsH GLACIAL Bens. 
a, Pecten islandicus (Miill.) (4); b, Leda truncata (Brown) (4); ¢, Leda lanceolata (Sow.) 
(Yoldia arctica, Mill.) (3); d, Tellina lata (Gmelin) (T. calcarea, Wahl.) (4); e, Saxi- 
caya rugosa (Pennant) (4); 4 Natica clausa (Brod. and Sow.) (4); g, Trophon 
scalariforme (Gould) (T. clathratum) (4). 
level from which they have yet been obtained is 524 feet, in one of the 
interstratifications in the boulder-clay at the Lanarkshire locality just 
referred to. Subsequent elevation of the land has brought up within 
tide-marks some of the clays deposited over the sea-floor during the 
time of the submergence. In the Clyde basin and some of the western 
fjords these clays (Clyde beds) are full of shells. Comparing the species 
with those of the adjacent seas, we find them to be more boreal in 
character ; nearly the whole of the species still live in Scottish seas, 
though a few are extremely rare. Some of the more characteristic 
northern shells in these deposits are Pecten islandicus, Tellina lata 
(T. calcarea), Leda truncata, L. lanceolata ( Yoldia arctica), Saxicava rugosa, 
Panopea norvegica, Trophon scalariforme (T. clathratwum), and Natica 
clausa (Fig. 426). 
