Spore cones of horsetail trees

Spore cones of horsetail trees bear sterile leaflets (bracts) in whorls.
The sporangia sit between these whorls. Two types are distinguished based on the way in which these sporangia are attached to the stem:
Palaeostachya , which has the sporangia in the axils of the bracts
Calamostachys, which has the sporangia at right angles between the whorls

Palaeostachya
(drawings after Stewart & Rothwell 1993)     

Calamostachys

In most cases it is difficult or impossible to identify the type of a cone.
Still attached spore cones of a horsetail tree.
Sobernheim (Germ). Lower Permian. Length of the long cone 5 cm.

Palaeostachya sp.
Spore cone of a Calamites-tree.
Piesberg. Westfalian D.
Height of the photo 6 cm.

Palaeostachya sp.
Several cones at one axis,
Piesberg. Westfalian D.
Height of the photo 15 cm.

Calamostachys binneyana
Microphoto. The arrow indicates an
attachment of a sporangium. The
sporangium itself is not present.
Ruhr district (Germ.). Namurian C.

Calamostachys sp.
Piesberg. Westfalian D.
Height of the photo 4 cm.