XXVIl] MARIOPTERIS, ETC. 533 



of fertile specimens. As examples of designations based pri- 

 marily on the branch-system of compound leaves, the genera 

 Mariopteris, Diplotmema, and Palmatopteris may be briefly 

 considered (fig. 354 A — C). Dr Kidston^ is of opinion that the 

 creation of new genera for purely vegetative characters of fronds 

 is of no real advantage, and he prefers to retain the older 

 provisional names for species known only in the sterile condition. 

 On the other hand, if we are sufficiently familiar with specimens 

 large enough to enable us to recognise a well-defined morpho- 

 logical character, it may serve a useful purpose to employ a 

 generic designation for features which may have a phylogenetic 

 value. A comparative examination of Palaeozoic, Mesozoic, and 

 recent compound fronds, including both Pteridosperms and true 

 ferns, brings to light certain distinguishing features character- 

 istic of the older types which, as Potoni^ maintains'', point to 

 the derivation of the pinnate habit from a primitive dichoto- 

 mous system of branching. For a more complete discussion of 

 this question reference should be made to Potoni^'s suggestive 

 papers. Among recent ferns Matonia and Dipteris, two 

 survivals from the past, afford instances of fronds with a 

 branching system of the dichotomous type. 



Similarly, in Oleichenia, Lygodium, and more rarely in 

 species of Polypodiaceae (e.g. Davallia aculeata, fig. 232) 

 dichotomy is a striking feature of the fronds. In the great 

 majority of recent ferns the fronds have assumed a piimate 

 habit. Among Palaeozoic fern-like fronds dichotomous branch- 

 ing of the main rachis and of the pinnae is much more common. 

 Potoni6 draws attention to several other features which 

 distinguish Palaeozoic fronds from the majority of later 

 species: the frequent occurrence of pinnules borne directly on 

 the main rachis (fig, 354, D), and of modified pinnules or 

 Aphlebiae on the rachis and petiole, are characters to which he 

 attributes an evolutionary significance. The main point is that 

 a comparative examination of leaf-form affords evidence in 

 favour of the view that the modern type of frond, with its 

 naked rachis bearing two rows of pinnae, has been derived from 

 a less specialised type in which the distinction between the 



> Kidston (012) p. 191. a PotonW (95) ; (99). 



