334 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



The second spermophytic Lepidophyte type, known as Miadesmia 

 and coming from the Lower Coal Measure of England, was borne on 

 a slender herbaceous form. Each sporophyll bore a ligule like those 

 of the vegetative leaves and had a fringed lamina. The megaspor- 

 angium was attached to the proximal adaxial face of the sporophyll 

 and developed a single megaspore, which filled its cavity, and was 

 inclosed in an integument arising from the proximal region of the 

 sporophyll, which completely invested the megasporangium, except 

 for a circular micropylar opening at the beak-like distal apex of the 

 sporangium. The micropyle was surrounded by numerous integumen- 

 tal hairs or tentacles, which probably facilitated fertilization. Some 

 megaspores are filled with prothallus. The whole sporophyll was 

 shed at maturity. In some respects Miadesmia was more advanced 

 than Lepidocarpon, and while neither are morphologically homolo- 

 gous with the seeds of the other spermophytes, they are physiologic- 

 ally true seeds, which apparently did not evolve further along lines 

 leading to higher plants, as did the seeds of the Pteridospermophytes. 



It can not be said that there is any unanimity of opinion regard- 

 ing the phylogenetic position of the Lepidophyta. Some students 

 regard their origin as entirely independent of the Arthrophyta, 

 while others regard them as probably distantly related. to the pro- 

 Sphenophyllum stock that represented the Arthrophyte ancestral 

 line. At the other end of the phylum, there are students (Seward, 

 Potonie) who consider that the Araucariales and perhaps the bal- 

 ance of the Coniferophytes diverged from the Lepidophyte stock, 

 while others justly doubt that the phylum had any higher issue. 



The view advocated here, an opinion frequently advanced, is that 

 the Lepidophyta represent a group of forms derived by reduction 

 from more megaphyllous ancestors, and that the prevailingly simple 

 relationship between the sporangia and the sporophylls is due to 

 simplification or reduction from sporangiophoric ancestors closely 

 related to the theoretical pro-Sphenophyllum type. This view is based 

 in part on a consideration of the paired sporangia of Canthelio- 

 phorus with their central sporangiaphoric plate, on the ventral pad 

 of Spencerites, the core of sterile tissue in Mazocarpon, the sterile 

 plates in various Lepidostrobus sporangia, and the stalked sporangia 

 of the modern Psilotales. A consideration of the details of mor- 

 phology and the fossil record, too extensive a subject for presenta- 

 tion in the present brief review, leads to the conclusion that the 

 Lepidophytes are not related to any of the higher seed plants and 

 never gave rise to more highly organized types. 



PHYLUM PTEmiXXSPERMOrHYTA. 



The recognition and partial elucidation of the seed ferns of the 

 Paleozoic is one of the outstanding paleobotanical achievements of the 



