244 
THE CRETACEOUS ROCKS OF BRITAIN. 
Ammonites bocliumensis 
„ Mantelli 
Nautilus subradiatus 
Pleurotomaria perspectiva 
Cyprina ligeriensis 
,, quadrata 
Pecten asper 
„ orbicularis 
Lima simplex 
Spondylus striatus 
Exogyra conica 
„ haliotoidea 
Ostrea canaliculata 
,, frons 
„ phyllidiana 
„ vesiculosa 
Rhynchonella compressa 
„ grasiana 
Cidaris vesiculosa 
Ditrupa difformis 
A little further north is Boussieres, where similar beds occur, 
and are overlain by marl containing Bel. plenus. The section in 
one quarry at this place is as follows : — 
ft. in. 
5. White sandy marl with glauconite grains - - - 13 0 
4. Bluish-grey marl --------66 
3. Glauconitic marl with Bel. plenus and many sharks’ 
teeth - - - - - - - - - -10 
2. Bright green marl with many fossils at the base (see list 
below) - ------- 73 
1. Thin layer of marl with grains of limonite and glauco¬ 
nite, resting on- - - - - - - 03 
Devonian limestone. 
28 0 
The thin layer at the base is analogous to that in the same posi¬ 
tion at Sassegnies, but as there is no sand above, Professor Barrois 
considers the “ glauconie d’Avesnes ” to be absent, and regards 
No. 2 as the zone of Ammonites laticlavius, No. 3 being that of 
Bel. plena. But Am. laticlavius does not occur, and we do not 
think that satisfactory results will ever come from the endeavour 
to find the zones of Wissant in the thin and variable set of Tourtias. 
The Tourtia must be regarded from a more general point of 
view. That many of its fossils are species which only occur at the 
base of the Cenomanian in the deeper parts of the Anglo-Parisian 
basin is only what might lie expected, for such species would migrate 
she rewards as the water became too deep for them. Hence we can 
see no reason why Nos. 1 and 2 of the above section should not be 
regarded as a littoral representative of some higher part of the 
Lower Chalk. 
The accompanying diagram (Fig. 53) will assist the reader to 
comprehend the lithological changes which have been described 
ip the preceding pages. 
