from the Coralline Crag. 97 



therefore called an older pliocene deposit, and presumed to 

 be contemporaneous with all other formations exhibiting the 

 same proportion of extinct Testacea ! Terebratula variabilis 

 would, of course, be one among the number ; and it was not 

 for M. Deshayes to enquire into the conditions attending its 

 occurrence. He had simply to decide upon the number of 

 recent species in the shells submitted to his examination from 

 a given deposit, and then to assign its geological position by 

 the new principles of chronological classification. 



In this instance, however, the result obtained from the 

 application of the per-centage test has been completely in- 

 validated, by the discovery of facts which indicate an asso- 

 ciation of the fossils of different periods over one deposit ; 

 clearly pointing out the extreme caution with which those 

 general conclusions should be received which are based upon 

 the numerical calculations of the conchologist. Had the crasr 

 occupied some very distant locality, and our acquaintance 

 with its history been limited to the examination of its fossils, 

 hastily gathered by some geological explorer, as is the case 

 with many tertiary deposits, it would have been called an 

 older pliocene deposit, and the accuracy of this conclusion 

 would never have been questioned. 



The facts which have been brought forward respecting the 

 history of the Suffolk crag, and which indicate such wide 

 limits of error in the application of the per-centage test, have 

 only been elicited in consequence of long continued and care- 

 ful investigation. No one source of fallacy connected with 

 numerical calculations has there been pointed out, which may 

 not also exist to a greater or less extent in all other tertiary 

 formations. 



To return, however, from this digression to the immediate 

 object of my paper, I may observe that the Terebratula va- 

 riabilis, like the Voluta Lamberti, is unknown in that part of 

 the crag which contains mammalian remains. The specimens 

 figured are from a quarry at Sudburn, upon the estate of 

 the Marquess of Hertford : I have also found it in the large 

 quarry by the side of the road leading from Aldborough 

 to Leiston, and it is of frequent occurrence at Ramsholt. 

 Could Mr. Sowerby have seen this fossil in the state in which 

 it is procured at these localities, I do not think he would 

 have distinguished it by the term variabilis. Maxima would 

 seem a more appropriate designation ; particularly too, if, as I 

 have some reason to imagine, the variations in form are prin- 

 cipally confined to the early periods of growth. 



