254 <A. J. Jukes-Browne—The Subdivisions of the Chalk. 
Ventriculites are also abundant; and there is a new species of 
Rhynchonella (R. reedensis), which may be regarded as intermediate 
between Rh. Cuviert and Rh. plicatilis. 
The rock consists of hard yellowish semi-crystalline chalk or 
rather limestone, and there are sometimes two distinct beds separated 
by a band of soft pulverulent chalk. Its upper surface is clearly 
defined and occasionally presents the appearance of having suffered 
erosion. As it is traced northward, however, it appears to lose some 
of these distinctive characters, and to be represented by one or more 
layers of hard yellowish chalk, containing only a few of the 
characteristic fossils: it occurs with this facies in certain quarries 
along the high ground between Balsham and Newmarket. 
10.—Zone of Micraster cor-bovis.—Above the Chalk Rock there is 
soft white chalk with many layers of flints. In this Micrasters are 
particularly abundant, and some of those found at Balsham are 
considered by Dr. Wright to belong to M. cor-bovis, Forbes, ‘but they 
are associated with others which greatly resemble the form known 
as cor-testudinarium, and this is undoubtedly the zone in which 
Dr. Barrois finds the latter to be generally abundant. 
The other fossils common here are Ananchytes ovata, Cidaris 
sceptrifera, Spondylus spinosus, Rhynchonella plicatilis, and Terebratula 
semiglobosa, var. bulla of Sowerby. Many of the last-mentioned are 
exceedingly like 7. carnea of the Norwich Chalk, and they may be 
regarded as intermediate between T. semiglobosa and T. carnea, 
which is an interesting fact when their intermediate position in the 
geological series is also taken into account. 
Beyond this zone our observations did not extend, and the 
remainder of the Upper Chalk in its extension through Essex is: 
much obscured by the superjacent glacial deposits, but the succes- 
sion of zones could probably be made out by working along some of 
the valleys. 
The following is a summary of the principal facts concerning the 
Cambridgeshire Chalk. 
1. That the zones of life are similar to those established else- 
where, and have been actually traced over a considerable area. 
2. That there are two breaks of more or less importance, marked 
by the occurrence of the hard rocky beds which are termed the 
Melbourn Rock and the Chalk Rock. 
3. That a middle division of the Chalk is thus marked off, and 
that its establishment is confirmed by the paleontological evidence. 
Many facts in support of the last conclusion are given in the 
memoir before referred to, and though complete data for a com- 
parison of the three faunas do not yet exist, yet it is quite possible 
to form an approximate estimate. In making such calculations it is 
best to consider the Invertebrata only; of these about 80 species 
have been obtained from the Lower Chalk near Cambridge, and no 
less than 70 of them appear to have died out before or during the - 
deposition of the Melbourn Rock, only 10 of them having yet been 
found in the Middle Chalk. Further explorations may result in the 
discovery of a. few more lingerers in the higher beds, but this will 
