Aisle Jukes-Browne—The Subdivisions of the Chalk. 255 
not alter the inference which the numbers clearly indicate, viz. that 
a very important break occurs in the succession of life at this time. 
Moreover, similar calculations based on the lists of fossils found in 
other areas by Caleb Evans, Dr. Barrois and F. G. H. Price, strongly 
confirm this conclusion, for in no case are more than three of the 
recorded species common to the two assemblages. 
The great difference between the two faunas is well illustrated by 
the amount of specific and even generic change exhibited in certain 
classes of animals, such as the Echinodermata; thus out of 20 
species recorded by these authors as occurring in the Lower Chaik, 
only Goniaster mosaicus and two species of Cidaris range up into the 
Middle Division, whence about an equal number of species have 
been obtained. The genera Peltastes, Pseudodiadema, Hchinocyphus 
and Hemiaster do not appear in the newer fauna, while Cyphosoma, 
Galerites and Micraster are absent from the older. 
The Ammonites again are represented by different species in these 
two divisions of the Chalk, only one (Am. Lewesiensis) being yet 
known as occurring above and below the thin band of marl which 
separates the Turonien from the Cenomanien in the South of 
England. 
It is clear, therefore, that a distinct paleontological break occurs 
at the horizon above indicated, and that a large number of species 
became extinct or migrated to other areas. Now such a result may 
be produced either by a sudden change in the conditions of existence, 
or by a gradual change effected during a period of time that was 
unfavourable to deposition. As far as we can tell, the conditions of 
existence do not seem to have changed very greatly or very suddenly, 
for the beds of chalk above and below this horizon are not very dis- 
similar in lithological character, and the differences are hardly 
sufficient to have caused much alteration in the forms of life: they 
are not in fact so great as the differences presented by the lower and 
upper portions of the inferior division itself. We cannot but con- 
clude, therefore, that the break between the two formations marks 
the lapse of a considerable period of time. 
It only remains for me to indicate how far these divisions are 
likely to be similarly developed in Norfolk; in limine I shouid 
observe that in proceeding from Cambridge to Norfolk it is probable 
that we pass from one area of deposition to another ; at any rate, the 
conditions which prevailed in the latter during the earlier stages of 
the formation were very different from those which obtained in the 
London Basin. Until therefore we know more of the manner in 
which one facies of the Lower Chalk passes into the other, no 
attempted correlation can be more than suggestive, and I only offer 
the following remarks as a kind of commentary on the record of facts 
and fossils given by Dr. Barrois (op. cit. p. 156, et seq.). 
In the Hunstanton section it is clear that the zones of the great 
assise @ Holaster subglobosus have very diminutive representatives, 
and I doubt whether the term Chalk Marl is applicable to any of 
them. There is some probability in Dr. Barrois’ suggestion that 
the “Sponge bed” is the analogue of the so-called Chloritic Marl, 
