290 STRATIGRAPHICAL GEOLOGY 



impossible to distinguish the sterile fronds of these ferns from 

 those of the Pteridosperms ; thus some of the fronds known as 

 Pecopteris and Sphenopteris belong to one group and some to the 

 other, so that these names only stand for certain forms of sterile 

 fronds which are similar in their venation and general characters. 



In the present state of our knowledge, these names have still to 

 be used generically, and some of the commonest frond-forms thus 

 designated are Alethopteris (Fig. 96, ft), Neuropteris (Fig. 95, 4), 

 Odontopteris (Fig. 95, 3), Pecopteris, Cardiopteris, Linopteris, Mari- 

 opteris, and Sphenopteris (Fig. 95, i and 2). 



The seeds of the Pteridosperms have long been known under 

 the names of Lagenostoma, Trigonocarpus, Rhabdocarpus, etc. 



Cordaitales. The stems of plants in some respects resembling 

 the true Conifer* (Pitys and Dadoxylori) have also been "met with, 

 especially in the volcanic tuffs of the Scotch Carboniferous Series. 

 The lower slopes of the old volcanoes were doubtless clothed with 

 these trees, their habitat being in such high and dry localities, and 

 not in the swampy ground where the other plants flourished. In 

 the structure of their stems these plants show affinities to the 

 Pteridosperms as well as to the Coniferee. 



Cordaites was a remarkable plant placed by most botanists in a 

 distinct phylum, but by some with the conifers. The stem grew to 

 a height of 20 or 30 feet, and carried a number of long sword- 

 shaped leaves with parallel venation, something like those of a 

 yucca. The leaves were attached by broad bases and left large 

 leaf-scars on the stem when they fell off. The tree bore bunches 

 or spikes of fruit (Antholites) with seeds somewhat resembling the 

 fruits of a yew, but more heart-shaped, and these are known as 

 Cardiocarpus. 



Some of the plants above mentioned are more abundant in the 

 lower part of the series and some in the higher parts. A study of 

 their distribution has enabled Dr. Kidston x to recognise four 

 successive assemblages of plants in British Coal-measures, and Zeiller 

 has found the equivalents of all but the uppermost in those of the 

 continent. The principal features of these assemblages are given 

 below. 



Plants of the Millstone Grit and Lower Coal-Measures 



Lepidodendra are abundant ; Sigillaria not very common. 

 Calamites are represented by 0. Suckovi and G. ramosus. The 

 prevalent fern forms are Alethopteris and Sphenopteris ; common 

 species of the first being A. lonchitica and A. decurrens, of the 

 second S. obtusiloba. 



