A. J. Jukes-Browne—The Subdivisions of the Chalk. 258 
small round quartz pebble was also found in this stratum at the 
Shelford pits, near Cambridge. 
Dr. Barrois observed the occurrence of a yellow marly band at the 
top of the Cherry Hinton quarry, and calls it the zone of Belemnites 
plena in a remanié state.’ I afterwards found that this was the 
basement bed of the rock so well exposed at Shelford and Melbourn, 
and that the overlying beds noted by Barrois, and referred to his 
zone of Inoceramus labiatus, were really part of the same rocky band, 
which I now regard as forming the base of the Turonien or Middle 
Chalk of Cambridgeshire. 
It cannot be regarded as in any sense a paleontological zone, but 
rather as a line of demarcation between two distinct groups of strata, 
and separating two different fossil assemblages ; nearly all the 
species found in the Lower Chalk have now disappeared, and only 
two or three cosmopolitan forms survived to reappear in the beds 
above, which contain an entirely new assemblage of Brachiopods and 
Echinoderms. 
6. Zone of Rhynchonella Cuvieri.—The Chalk of this zone is not 
often quarried, but the exposures that do occur show it to be very 
evenly bedded, white, and rather hard; its thickness, including the 
Melbourn Rock at the base, is about 70 feet. Scattered flints are 
occasionally found in the upper part of the zone. Its characteristic 
fossils are Inoceramus mytiloides (= labiatus) and Rhynchonella 
Cuwiert; Echinoderms are rare, but Galerites globulus, Cidaris 
hirudo, and C. dissimilis have been found. 
7. Zone of Terebratulina gracilis—At the base of this division 
there are some layers of grey marl containing small Brachiopods, 
and some beds of hard compact chalk. Flints occur at certain 
horizons, but most of them are remarkably elongated and root-like, 
and are very different from the nodular flints of the Upper Chalk. 
Terebratulina gracilis is everywhere abundant, as is also Galerites 
subrotundus, and Discoidea Dixoni is occasionally found; most of 
the fossils in the zone below range up into this; its full thickness 
cannot be far short of 100 feet, but its upper limit has not been 
fixed with any certainty in Cambridgeshire. 
8. Zone of Holaster planus.—The chalk of this zone is soft and 
white, and contains regular layers of black flints, some long and 
narrow, others large and nodular. Its thickness is probably 50 or 
60 feet, and its fauna is quite different from that of the zone below ; 
Holaster planus makes its appearance, together with a species of 
Micraster (? breviporus) and other forms that become common in the 
Upper Chalk, such as Cidaris sceptrifera, Cyphosoma radiata and 
Spondylus spinosus. Ventriculites too are especially abundant in this 
zone, and serve to distinguish it from the zone below. 
9. Chalk Rock.—This is really the topmost bed of the zone above 
described,—the same olaster and Jicraster are found in it, 
together with many other fossils which appear to have a similar 
range, such as Ammonites Prosperianus. It is specially characterized 
by the occurrence of univalves, Turbo gemmatus being the commonest. 
1 Recherches sur Terr. Cret. Sup. p. 155. 
