366 
THE CRETACEOUS ROCKS OF RRITAlN f . 
After Terebratulina, the commonest and most characteristic fossil 
of this zone is Galerites subrotundus. Inocemmus mytiloides 
ranges up into it, and a larger species, the Inoc. Cuvieri of Sower by, 
is also common. Spondylus spinosus is another fossil of frequent 
occurrence. Holaster planus makes its appearance in the higher 
beds, and is associated with Micraster coibovis and M. Leskei. The 
Ammonites which occur in this zone are Am. [ Prionotropis ] Wool- 
gari, Am. [ Pachydiscus ] peramplus, and a form which is gen¬ 
erally known as Am. [ Haploceras ] lewesiensis, but may be a 
different species from that occurring in the Lower Chalk; none 
of them are of common occurrence. Fish remains are common 
in some places, and Ventriculites are of frequent occurrence in the 
higher beds. 
In the south-eastern and midland counties the thickness of the 
Terebratulina zone is from 130 to 160 feet, but westward it thins 
greatly till in some parts of Dorset it is not more than 50 feet 
thick. Northward also from Cambridgeshire it becomes rapidly 
thinner, and in Norfolk it is apparently only about 60 feet thick; 
in Lincolnshire, however, it is thicker again, and is well developed 
in Yorkshire, exhibiting a great thickness of flint-bearing chalk 
in the cliffs near Speebon. 
Comparative tables showing the development of the Middle Chalk 
in various parts of England will be found on pp. 409 and 558. 
Fossils of the Middle Chalk. 
The fauna of the Middle Chalk differs very considerably from 
that of the Lower Chalk. Very few species range up from the 
Lower Chalk into the zone of Rhynchonella Cuvieri, and though 
some do reappear in the Chalk Bock (zone of Holaster planus), 
the statement made in 1880* still remains true — namely, “ that 
a distinct palaeontological break occurs at the horizon above men¬ 
tioned, and that a large number of species became extinct or mi¬ 
grated to other areas.” Perhaps the most noticable feature of 
the Middle Chalk fauna is the incoming of the genus Micraster ; 
in Devonshire two species of this genus are not uncommon in the 
lower part of the zone of Rhynchonella Cuvieri, and Micraster 
corbovis is not uncommon in the higher part of the Terebra- 
tulina zone in Kent, Sussex, Cambridgeshire, and Suffolk. These 
and the other English species of Micraster have been recently 
carefully distinguished and described by Dr. A. W. Bowe.f 
On the whole the Middle Chalk is not rich in fossils, though in¬ 
dividual species like Rhynchonella Cuvieri, Terebratulina lata, 
and certain species of Inocemmus are often abundant. 
Reptilia.— So far as we are aware few Beptilian remains have 
been recorded as coming with certainty from the Middle Chalk, 
but there is no reason to doubt that such remains occur in this part 
of the Chalk. Cimoliosaurus Bernardi, for instance, is recorded bv 
* Geol. Mag., Dec. 2, Yol. vii., p. 255. 
f See Quart. Journ. Geol. Soc., Yol. lv-, p. 494. (1899.) 
