262 =A. J. Jukes-Browne—The Subdivisions of the Chaik. 
are most abundant in its lower portion, which is everywhere 
characterized by the frequent occurrence of Molaster subglobosus. 
Holuster trecensis, Discoidea cylindrica, and Ammonites rhothomagensis 
may also be mentioned as occurring in this zone, but most of 
the other fossils range up from the beds below. elemnites plenus 
only occurs in the topmost bed. 
So far we have found a continuous succession without any marked 
break either in the series of beds or of organic remains. All the 
fossils of the Chalk Marl range into the Totternhoe Stone, and the 
number of forms that are confined to the latter will probably be 
largely diminished by future explorations in the overlying zone, 
with which it is already known to have many species in common. 
The three subdivisions may, in fact, be regarded as forming a group 
of beds characterized by a fauna of one general type, and they 
are the exact equivalents of the Chalk Marl and Grey Chalk at 
Folkestone, as defined by Mr. F. G. H. Price.! 
Dr. Barrois has proposed a similar arrangement for this portion 
of the series in the South of England; he considers it as forming the 
“ Assise du Holaster subglobosus,” which he subdivides into four 
niveaua or subsidiary zones; the first is that usually called Chalk 
Marl, the second (niveau de Am. varians) corresponds roughly with 
our Totternhoe Stone,* the third is certainly co-ordinate with the 
greater part of our Holaster subglobosus zone, the upper portion 
of which may represent Barrois’ fourth zone, that of Belemnites 
plenus. 
This last zone has a greater development in France, and Dr. 
Barrois suggests that it has suffered considerable erosion in England, 
and observes that the basement bed of the Turonien frequently 
contains rolled fragments of Belemnite. In this he anticipates the 
conclusion to which I have been led by a study of the Cambridge 
section, for I have little doubt that at this point in the series there is 
a distinct stratigraphical and paleontological break, as will appear 
in the sequel, and is more fully discussed in the Geological Survey 
Memoir. ; 
5. Melbourn Rock.—This rocky belt, or niveau as a French geologist 
would call it, is never more than ten feet thick, but it presents such 
marked and constant lithological charactersas to deserve a distinct local 
denomination. Excellent sections of it being exposed in two quarries 
near Melbourn, between Cambridge and Royston, it has been 
named after that village. It consists of several thin courses of hard 
yellowish rocky chalk, separated by layers of greyish laminated marl 
or shale, generally containing numerous nodules or pebbles of hard 
chalk, to which small Oysters are frequently attached. 
Besides this oyster, which is a variety of Ostrea vesicularis, crushed 
Rhynchonelle and rolled Belemnites also occur, together with Inocer- 
amus labiatus and Fish teeth, Zamna, Otodus, Saurocephalus, ete. A 
' Quart. Journ, Geol. Soc. vol. xxviii. p. 445. : 
* While bearing testimony to the great value and general accuracy of Dr. Barrois’ 
work, I am constrained to point out that he never correctly identified the position 
of the Totternhoe Stone, having first placed it too high, and afterwards too low 
in the series, 
