MIDDLE CHALK—GENERAL ACCOUNT. 361 
that it should be most common within such limits ; thus we are 
not obliged to include the chalk below the. Chalk Rock in this zone 
just because Holaster planus occurs in it. 
Mr. H. Woods has pointed out* that the Chalk Rock facies of 
this zone would be more conveniently indicated by the name of 
Heteroceras reussianum, a species which is widely distributed and 
has been chosen as the index for the zone in Germany. The only 
objection to this is the great rarity of the Heteroceras in France 
and the South of England. 
But, having defined the zone of Hoi. planus as that where this 
fossil is generally most abundant, and is associated with the fauna 
of the Chalk Rock, we are next faced by the question whether this 
zone should be included in the Middle Chalk or not. The first 
consideration should unquestionably be given to the fauna, and 
this has been carefully studied by Mr. H. Woods. Of the 56 species 
of Cephalopoda, Gasteropoda, and Lamellibranchiata examined by 
him, no fewer than twenty-five are at present only known in the 
Chalk Rock beds ; of the remainder nine or ten have an exclusively 
downward range, and the only two which range up and not down 
are Lima granosa and Lima. Hoperi. Of Brachiopods there are 
only two which are specially Senonian —viz., Terebratula carnea 
and Rhynchonella odoplicata. Of Echinoderms eight species are 
mentioned by Mr. Wood, and two others are now known to 
occur : of these four range both up and down, four are species 
which only range upwards ( Cardiaster ananchytis, Echinocorys 
vulgaris, Micraster prcecursor and M. cortestudinarium). The 
two other species mentioned by Mr. Woods, I regard as of doubt¬ 
ful occurrence in Chalk Rock ; these are Galerites albogalerus , 
very rare and possibly a rather flattened specimen of G. subrotun- 
dus, and a doubtful specimen of Cyphosoma spatuliferum. 
Mr. Woods remarks that “on the whole I am inclined to think 
that the affinities of the fauna are closer to the Turonian than to 
the Senonian.” I am of the same opinion; moreover, E. Hebert, 
Professor Barrois, and other French geologists had previously 
come to the same conclusion, and of late years it has been usual 
in France to include the zone of Holaster planus in the Turonian. 
Are we, then, to place the upper limit of our Middle Chalk at the 
top of this zone of Hoi. planus or Heteroceras reussianum ? Unfor¬ 
tunately the zone has no definite top which can be followed for 
any great distance, for beds having the lithological character of 
the Chalk Rock occur at various horizons above the lowest and 
most fossiliferous bed, while in some districts there are no such 
rock-beds at all. Neither is there at any horizon such a marked 
change of fauna as would enable us to be sure .when we passed 
from one zone to another, except in those districts where typical 
Chalk Rock is well developed and fossiliferous. 
There is, in fact, a very close relationship between the zones of 
Holaster planus and Micraster cortestudinarium; so close, indeed, 
* Quart. Journ. Geol. Soc., Vol. lii., p. 70, (1896). '. 
