of the White Chalk. 337 



by the Ventriculida found respectively in each, and which — as I 

 shall particularly show when detailing the species of that family, 

 — belong, in the three beds, to three distinct and strongly-marked 

 groups, a fact which illustrates the importance of the careful in- 

 vestigation of any single family, to whatever class it may belong. 

 Of the three groups into which I shall show that that family must 

 be divided, the first ( Ventriculites) is found in the upper chalk ; 

 extremely rarely any species of it, and then very doubtfully, in the 

 middle chalk : the second (Kephalites) is very rarely, and then 

 also doubtfully, found in the upper chalk : I have never seen a 

 single specimen of either from the lower chalk. Of the third 

 group (Brachiolites), while some species are perhaps found in the 

 upper chalk and the same are found in the middle, some of the 

 most marked species — differing, with one exception, and even in 

 that with a modification of character, from those last named — 

 are found in the lower chalk and even greensand. 



I have remarked that some species of Ventriculidce are found 

 doubtfully in one or the other bed. This leads me to notice, 



II. The causes of the doubt and confusion which exist as to 

 the true position of many fossils from the chalk, and conse- 

 quently as to the character of the respective geological seras. 



From the particular mode in which the denudation of the chalk 

 has taken place, it happens that, in many pits, in many parts of 

 Kent, the upper part of the same pit exhibits the upper chalk, 

 the lower part the middle chalk, while, within five minutes' walk, 

 lies the deep lower chalk. Now whether fossils be picked up on 

 the floor of such a pit or out of blasted blocks, it will, generally, 

 be equally impossible, from the mere hand specimen, to deter- 

 mine to which bed it belonged. It is in such pits as these that 

 the problematical Ammonites, &c. have been found ; and it is in 

 such pits as these that I have picked up the specimens of Ven- 

 triculidse which I have above assigned as doubtful. It is only by 

 very careful and personal examination of individual sites, and not 

 merely of localities, that the exact and accurate determination 

 can be made. 



Another important source of error is the faults and disloca- 

 tions which have taken place in the chalk*. These, in rocks so 

 similar as the different beds of the white chalk, escape any 

 but the closest attention. The following instance is instructive. 

 In the course of the last spring I visited, in company with 

 Mr. Morris, the collection of Mr. Harris at Charing. I learned 

 that there had long been a friendly dispute between Mr. Harris 



* I do not allude to such faults as Mr. Lyell figures, p. 27. vol. ii. of the 

 1 Elements,' and which can never mislead, hut to cases where white chalk 

 only is seen on both sides, with nothing to distinguish the two to the super- 

 ficial observer. 



Ann. $ Mag. N. Hist. Vol.xx. 24 



