FOSSIL MOLLUSC A. 



293 



may be the junction of the two formations. This 

 stratum lies in a denuded hollow of the harder and 

 more consolidated strata of the Coralline Crag. From 

 Thorpe Common, near Aldborough, the true Norwich 

 Crag extends to the city whence it derives its name. 

 Its greatest thickness is about eight to ten feet. This 

 crag is distinguished by its abundance of recent forms, 

 and the general absence of southern types of shells, 

 thus showing that the cold was increasing. The total 

 number of species of moUusca catalogued from it is 

 one hundred and forty, of which one hundred and 

 twenty-three are still living ; seventeen are supposed 

 to be extinct. Of the above number, one hundred 

 and one species are still living in British seas, twelve 

 are Arctic and North American, eight Mediterranean, 

 and two species Asiatic, the latter being Corbicula 

 fltinmialis and Paludina unicolor. From the fact that 

 twenty species of shells belonging to the Norwich Crag 

 are not found either in the Red or Coralline Crags, 

 Dr. Gwyn Jeffreys thinks that there is some difference 

 in their geological age, the Norwich Crag being more 

 recent than the Red Crag, and its shells of a more 

 arctic character. 



In 1865, the bed which had previously gone by the 

 name of the Norwich Crag was shown to be composed 

 of two deposits, usually separated by ten or fifteen 

 feet of sand, etc. The upper bed is distinguished by 

 a general absence of shore shells, such as Littorina 

 and Purpura, and by the presence of shells affecting 



