- 
874 STRATIGRAPHICAL GEOLOGY.  [Boox. VI. 
ceros leptorhinus, Trogontherium Cuviert, a horse and deer, likewise the 
living species of otter and beaver. Among the mollusca the follow- 
ing are characteristic forms: Paludina media, Hydrobia ventrosa, Turritella 
communis, Trophon scalariforme, Litorina litorea, Mytilus edulis, Nucula Cob- 
boldiz (Fig. 421), Cardium edule. One interesting feature is the decided 
mixture of northern species of shells, such as Bhynchonella psittacea, 
Scalaria greenlandica (Fig. 422), Panopxea norvegica, and Astarte borealis 
(Fig. 421). These may be regarded as the forerunners of the great in- 
vasion of Arctic plants and animals which, in the beginning of the 


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ad 
Fic. 421.—PLiocenr LAMELLIBRANCHS. 
a, Astarte borealis (Chemn.); b, Astarte Omalii (Laj.); ¢, Nucula Cobboldiz (Sow.); 
d, Congeria subglobosa (Partsch.) (8). 
Quaternary ages, came southward into Europe, together with the severe 
climate of the North. 
The Chillesford beds occur likewise as a thin local deposit 
chiefly in Suffolk. Among their organisms are Mya truncata, Mactra 
ovalis, Nucula Cobboldise, Cyprina islandica, Astarte borealis, Tellina obliqua. 
About two-thirds of the shells still live in Arctic waters. It is evident 
that, in these fragmentary accumulations of the Crag series, we have 
merely the remnants of some thin sheets of shelly sands and gravels laid 
down in the shallow waters of the North Sea, while that great lowering 
of the European climate was beginning which culminated in the suc- 
ceeding or Glacial period. 
The Forest-bed group comprises an interesting succession of beds, 
only a few feet in thickness, exposed for many miles at the base of the 

