S32 Norwich Geological Society. 



littoral shells in the Upper bed do not prove a shallow-water origin, 

 any more than occassional deep sea shells in the Lower Crag prove 

 deep sea deiDosition. " One swallow does not make a summer," 

 and in snch circumstances as these, we should be guided by the 

 general facts. Speaking of shallow-water shells in the Upper bed 

 I am reminded of a remark by the late Dr. Woodward, intended as 

 a warning to dredgers. He says, "It is important to distinguish 

 between dead and living shells, for almost every species is met with, 

 in the condition of dead shells, at depths far greater than those in 

 which it actually lived." The commonest shell in the Upper Crag 

 is Tellina ohliqua ; the nearest 'living representative of which 

 IS the Tellina proxima — a shell now living in thirty fathom water. 

 Another characteristic species of the Upper Crag, and one which 

 I venture to say is limited to it alone, is the Modiola modiolus, a 

 mollusck which lives in twenty to thirty fathom water. The Oyprina 

 Islandica is occasionally found in the Lower bed, but is more abim- 

 dant in colonies in the Upper, as at Postwick Grove. Bhynchonella 

 psittacea is also usually found in colonies in the Upper Crag, at Post- 

 wick Grove and Thorpe. This shell ranges in depth to as much as 

 one hundred and eighty fathoms, and is, therefore, par excellence, a 

 deep-sea mollusck. At Postwick it has been found in pairs, showing 

 that it must have lived near the spot. It is occasionally found in 

 the Lower Crag, but in single valves and waterwom, as though it 

 had been washed from some distance. The shells which inhabit 

 deeper water, and are Arctic or sub-Arctic in their character, which are 

 peculiar to the Upper Crag, (I have never found them in the Lower) , 

 are Astarte compressa, sulcata, and elliptica. Astarte horealis is some- 

 times found in the Lower bed, but in nothing like the quantities in 

 which it occurs in the Upper bed at Bramerton and Thorpe. Lucina 

 horealis is common to both beds. Venus fasciata is common in the 

 Upper bed at Thorpe, and belongs to a genus usually inhabiting 

 a depth of thirty to fifty fathoms. Cardium Groenlandicum is fre- 

 quently met with in the Upper bed, rarely in the Lower, and then 

 only the largest and strongest specimens. On the other hand, I 

 have met with them of all ages of growth in the Upper bed. This 

 shell is Arctic, and also lives in tolerably deep water. 



I have thus particularized the principal shells of the Upper Crag, 

 because they afford the strongest evidence of pure marine deposition, 

 and they also indicate a greater mean depth of water than the 

 shells of the Lower bed. 



Since Nucula Cohholdia is proved to be living, I know of but one 

 shell in the Upper Crag which is really extinct — Tellina ohliqua. 

 The other eleven, marked as extinct in the Norwich Crag, belong to 

 the Lower bed. There is, on the other hand, a decided greater per- 

 centage of species of Arctic or sub-Arctic shells in the Upper bed, 

 three species seeming to be peculiar to it. The proportion also of 

 these Northern shells is much greater in the Upper than in the Lower 

 bed. Here they occur in almost Northern percentage, and appear to 

 have been indigenous. 



Between the Upper and Lower beds there usually intervenes a 



