ECIIINOID GENUS ECHINOCOIÎYS

EXPLANATION OF PLATE 1.
(all figures V* natural size).
Here is shown in a general outline manner the development and probable relations
of the Belgian Echinocorys. Their possible origin are indicated bj the insertion of
smiic earlier species not found in the Senonian of Belgium. The stratigraphie relations
included here are naturally quite generalized and tiny particular form is not necessarily
restricted to the horizon which il OCCupieS here. In must cases, the level al which any
species is found on the diagram may represent either its earliest occurrence or its most
common horizon. The stratigraphie units are neither to scale nor proportional. Then
different thicknesses are only a result of the space required for figures during that
interval.
The upper Turonian, shown here as beds of Micraster leskei, is sometimes referred
to by various writers as beds of « M. breviporus ... « M. brrviporus »> is the young of
.1/. coranguinum and is encountered much higher stratigraphically. M. leskei is persistent
and reasonably constant in characters in the upper Turonian and should not he
easily confused with oilier Micrasters.
The level of the Coniacian here referred to as beds of Micraster decipiens has been,
and is still often miscalled beds of M. cortestudinarium. In 1878, Bayle (Foss. prin. des
Terr pi 156 figs. 1 & 2) separated the forms of England and France, which had been
commonly referred to the German .1/. cortestudinarium, under the name of M. decipiens.
Above this latter horizon would come the beds of M. coranguinum, Santonian, which
are not present in the Belgian sequence of chalks. At a higher level, Craie de Triviere
and Ohourg one finds a variety of M. coranguinum, M. coranguinum var. schroederi
which suggests that these chalks, the Triviere and Ohourg, represent either a chalk
horizon immediately above, or faintly related to the beds of M. coranguinum. Therefore,
for the most part the time of the deposition of the beds of M. coranguinum in adjacent
areas, is marked in Belgium by the gaps between the St. Vaast and Triviere and in part
by the Trivtere-Obourg gap.