While recognising a general resemblance to the sporophylls of Isoetes, certain differences are obvious: there is no ligule in the fossil leaves nor are there any distinct traces of vascular strands such as occur in the leaves of recent species. The form of the sporangium, more elongated than in the majority of recent forms, is compared by Saporta with that in a south European species Isoetes setacea Spr.
Such evidence as we have lends support to the inclusion of these Portuguese fossils in the genus Isoetites, but apart from the fact that we have no proof of any connexion between the stems and supposed sporophylls, the resemblance of the latter to those of Isoetes is, perhaps, hardly sufficient to satisfy all reasonable scepticism.
The generic name Isoetopsis was used by Saporta as more appropriate than Isoetes for some Eocene fossils from Aix-en-Provence which are too doubtful to rank as trustworthy evidence of the existence of the recent genus. The species, Isoetopsis subaphylla[151] is founded on impressions of small scales, 4 mm. long, bearing circular bodies which are compared with sporangia or spores.
Other records of fossils referred to Isoetes need not be described as they have no claim to be regarded as contributions towards the past history of the genus. Heer’s Miocene species Isoetites Scheuzeri and I. Braunii Unger[152] from Switzerland are based on unsatisfactory material and are of no importance.
Pleuromeia.
The generic name Pleuromeia, was suggested by Corda[153] for a fossil from the Bunter Sandstone, the original description of which was based by Münster[154] on a specimen discovered in a split stone from the tower of Magdeburg Cathedral.
The majority of the specimens have been obtained from the neighbourhood of Bernburg, but a few examples are recorded from Commern and other German localities: all are now included under the name Pleuromeia Sternbergi. Germar, who published one of the earlier accounts of the species, states that Corda dissented from Münster’s choice of the name Sigillaria and proposed the new generic title Pleuromeia. One of the best descriptions of the genus we owe to Solms-Laubach[155] whose paper contains references to earlier writers. Illustrations have been published by Münster, Germar[156], Bischof[157], Solms-Laubach and Potonié[158].
Pleuromeia Sternbergi. (Münster.)
[Fig. 134].
- 1842. Sigillaria Sternbergii, Münster.
- 1854. Sagenaria Bischofii, Goeppert[159].
- 1885. Sigillaria oculina, Blanckenhorn.
- 1904. Pleuromeia oculina, Potonié.
Pleuromeia Sternbergi is represented by casts of vegetative and fertile axes, but the preservation of the latter is not sufficiently good to enable us to draw any very definite conclusions as to the nature of the reproductive organs. Casts of the stems reach a length of about 1 metre and a diameter of 5–6 cm., or in some cases 10 cm.; all of them are in a more or less decorticated state, the degree of decortication being responsible for differences in the external features which led Spieker[160] to adopt more than one specific name.