2154 A TEXTBOOK OF THEORETICAL BOTANY 



1. Lyginopterideae 6. Protopityeae 



2. Rhetinangieae 7- Cladoxyleae 



3. Megaloxyleae 8. Medulloseae 



4. Calamopityeae 9- Aneimiteae 



5. Stenomyeleae 10. Cycadoxyleae 



The above groups are all based upon plants of which at least some part 

 of the anatomical structure is known. Side by side with this classification, 

 there must, however, be recognized a number of groups based upon frond 

 impressions alone, some of which are known to have been seed-bearing 

 and were therefore almost certainly Pteridosperms. Others bear no seeds 

 but may bear microsporangia which are not like Fern sporangia. All are, 

 on grounds of resemblance or of cuticular characters, regarded as probable 

 Pteridosperms. Only a few have actually been proved to have been borne 

 by plants of known structure and affinities. These " frond groups " are as 

 follows : 



1. Sphenopterideae 3. Alethopterideae 



2. Pecopterideae 4. Neuropterideae 



They are distinguished on their pinna-form and venation. Arnold, in his 

 " Introduction to Palaeobotany " (1947), uses a simpler and less compre- 

 hensive method, grouping the seed-bearing fossils into Lyginopteridaceae, 

 Medullosaceae and Calamopityaceae, to which are added two Mesozoic 

 families of recently recognized Pteridospermae— Peltaspermaceae and 

 Corystospermaceae — which seem to have been related to the Caytoniales. 

 He does not use the families of frond genera mentioned above. 



Gothan, in the second edition of the " Pflanzenfamilien " (1926), uses 

 two main families whose structure is fairly well known, under the names 

 Lyginodendraceae and Medullosaceae. Their author maintains that the 

 older name, Lyginodendron, was well founded by Williamson and is therefore 

 preferable to the newer Lyginopteris, now used by English authors. He 

 also continues to use the older name Cycadofilices for the sub-phylum as a 

 whole. 



The genera founded on stem structure he groups as follows: 



1. Steloxylaceae 5. Megaloxylaceae 



2. Cladoxylaceae 6. Calamopityaceae 



3. Cycadoxylaceae 7. Stenomyelaceae 



4. Rhetinangiaceae 8. Protopityaceae * 



While the frond genera are grouped into formal series thus: 



1. Archaeopterides 6. Odontopterides 



2. Sphenopterides 7. Neuropterides 



3. Pecopterides 8. Taeniopterides 



4. Alethopterides 9. Glossopterides 



5. Callipterides 10. Megalopterides 



* The two Mesozoic families had not been recognized at that date. 



