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Micrhystridium placophorum
Original description: [Valensi, 1949, p. 543-544] (translated from French):
Micrhystridium placophorum nov. sp.
Fig. 4.
Holotype: Lower Bathonian, Tour aux Cognons flint (Vienne).
Paratypes: Bajocian, Flée flint (V.); Lower Bathonian, Tour aux Cognons flint (V.).
The common characteristic of all representatives of this species is to possess an exactly spherical shell on the surface of which dark lines very clearly circumscribe pentagonal or hexagonal fields; in these fields there is always a darker differentiation with a punctuated and blackish surface, delimited by a more or less wide light band, of rounded outline and roughly parallel to that of the field which surrounds it, this characteristic combined with the shape of the processes allows to distinguish this species very easily from the previous one.
In the holotype (1, 2 of fig. 4) the upper face bears a pentagonal field, each angle of which gives rise to a process; other processes are seen on the edge of the shell; about fifteen in number, they are full, straight or slightly curved, moderately long and tapered.
The diameter of the shell is 10 μ, the processes are 3 to 4 μ long, the sides of the pentagons measure approximately 4 μ.
In the Bajocian paratype of Flée (3 of fig. 4) the same size, but the pentagonal field which occupies the upper face is smaller, its sides are shorter (2 to 3 μ), the central differentiation is relatively more important, and those of the processes which start from the angles of the pentagon are longer (4 to 5 μ instead of 3 μ in the holotype); the other processes are more numerous, which is due to the fact that the holotype was damaged during its fossilization; another similar individual, but damaged, was found in a Bajocian flint, from the Porleau rocks to the N of Poitiers.
The paratype of the Bathonian of the Tour aux Cognons (4 in fig. 4) is clearly larger (15 μ) and its processes are shorter.
All the individuals of this species are dark brown in color.
Microhystridium placophorum seems characteristic of the top of the Bajocian and the base of the Bathonian; however, I have not yet found it in Norman flints. A fairly similar species is Micrhystridium mendax Defl. from the Silurian, where the polygonal fields and spines are found;
but the (unique) holotype of this Silurian species shows no trace of the median differentiations of the fields, so characteristic of Michrystridium placophorum. Micrhystridium reticulatum Defl. (Cretaceous flint) also shows polygonal fields but is decorated with long hyaline horns and is, therefore, quite different.
Micrhystridium placophorum nov. sp.
Fig. 4.
Holotype: Lower Bathonian, Tour aux Cognons flint (Vienne).
Paratypes: Bajocian, Flée flint (V.); Lower Bathonian, Tour aux Cognons flint (V.).
The common characteristic of all representatives of this species is to possess an exactly spherical shell on the surface of which dark lines very clearly circumscribe pentagonal or hexagonal fields; in these fields there is always a darker differentiation with a punctuated and blackish surface, delimited by a more or less wide light band, of rounded outline and roughly parallel to that of the field which surrounds it, this characteristic combined with the shape of the processes allows to distinguish this species very easily from the previous one.
In the holotype (1, 2 of fig. 4) the upper face bears a pentagonal field, each angle of which gives rise to a process; other processes are seen on the edge of the shell; about fifteen in number, they are full, straight or slightly curved, moderately long and tapered.
The diameter of the shell is 10 μ, the processes are 3 to 4 μ long, the sides of the pentagons measure approximately 4 μ.
In the Bajocian paratype of Flée (3 of fig. 4) the same size, but the pentagonal field which occupies the upper face is smaller, its sides are shorter (2 to 3 μ), the central differentiation is relatively more important, and those of the processes which start from the angles of the pentagon are longer (4 to 5 μ instead of 3 μ in the holotype); the other processes are more numerous, which is due to the fact that the holotype was damaged during its fossilization; another similar individual, but damaged, was found in a Bajocian flint, from the Porleau rocks to the N of Poitiers.
The paratype of the Bathonian of the Tour aux Cognons (4 in fig. 4) is clearly larger (15 μ) and its processes are shorter.
All the individuals of this species are dark brown in color.
Microhystridium placophorum seems characteristic of the top of the Bajocian and the base of the Bathonian; however, I have not yet found it in Norman flints. A fairly similar species is Micrhystridium mendax Defl. from the Silurian, where the polygonal fields and spines are found;
but the (unique) holotype of this Silurian species shows no trace of the median differentiations of the fields, so characteristic of Michrystridium placophorum. Micrhystridium reticulatum Defl. (Cretaceous flint) also shows polygonal fields but is decorated with long hyaline horns and is, therefore, quite different.