96 Gigantic Species of Terebratula 



Another genus of Branchiopodous Mollusca, Lingula, now 

 only found in the seas of hot climates, is associated with the 

 Terebratula in the coralline crag. No legitimate inferences, 

 respecting the temperature of the period when this formation 

 was deposited, can be drawn from the latter of these two 

 genera, since, as I have already mentioned, it is one capable of 

 supporting various temperatures. 



The cliffs of Norfolk and Suffolk have suffered an amount 

 of degradation, within the historic period, which would lead 

 us to suppose that some hundred square miles, along this line 

 of coast, may have been swallowed up by the encroachments 

 of the sea, since our eastern tertiary deposits have occupied 

 their present level. Now, when we consider that the crag is 

 often little else than a mass of shells, having an average thick- 

 ness of perhaps 1 8 ft. or 20 ft., and that some thousand 

 individuals must often be contained in the space of a cubic 

 yard, it is easy to conceive that, could we only have access to 

 the deposits now forming in the neighbouring ocean, a diffi- 

 culty would sometimes arise, in endeavouring to distinguish 

 between those fossil and recent shells which may there have 

 been embedded during any very long period. In all proba- 

 bility we should meet with new forms, derived from the exten- 

 sive destruction of tertiary deposits, while we should also expect 

 to find some species of recent Mollusca with which we are as 

 yet unacquainted. In this investigation, an attention to spe- 

 cific distinctions would avail us nothing, and the shells of 

 both periods might have been so acted upon by external 

 agents as to have removed those adventitious characters which 

 each of them formerly possessed. Even now specimens of 

 Turbo littoreus are sometimes found in the crag of Norfolk 

 which exhibit scarcely any appreciable difference, when placed 

 beside dead shells of their existing analogue. I admit, however, 

 that the instances I have supposed would be exceptions to 

 the general rule, but it is otherwise with the Testaeea which 

 we find thus associated in the red crag. Here the organic re- 

 mains have indiscriminately acquired one common ferruginous 

 aspect, which has superseded ail other characters; and it is 

 utterly out of our power to determine whether the animal 

 belonging to these worn and solitary valves of the Terebratula 

 existed during the formation of both deposits, or only during 

 that of the coralliferous beds. 



Here, perhaps, it will not be irrelevant to notice the means 

 by which the age of the upper crag has been determined. 

 Ill species taken from it, collected at different times and by 

 different individuals, upon being examined by M. Deshayes, 

 were found to include 66 extinct forms; and the crag was 



