286 STUDIES IN FOSSIL BOTANY 



Many of these dimorphic fronds no doubt belonged 

 to Pteridosperms and not to Ferns. Thus a species of 

 Crossotheca (Fig. 109, F) has been shown by Mr. Kidston 

 to be the male fructification of Lyginodendron (Chapter 

 X.) ; Calymmatotheca is now known to represent a con- 

 dition of the seed-bearing apparatus of the same genus ; 

 Urnatopteris (Fig. 109, J) is probably, like Crossotheca, 

 a microsporangiate fructification. Some examples of 

 dimorphic fronds in plants which appear really to come 

 under the head of Ferns will be given in the next 

 chapter in describing the Botryopterideae. 



Before leaving the supposed Marattiaceous fructi- 

 fications, we may shortly refer to a genus which 

 combines Marattiaceous characters with those of other 

 families. This is the Sturiella of Weiss {Renaultia of 

 Stur). The structure of the leaf and sorus was worked 

 out by Renault J from silicified specimens. The frond 

 was oi the Pecopteris type, though its form is not 

 completely known, as the specimens were fragmentary. 

 The sori are ranged in two series on the pinnule, 

 along either side of its median nerve. Each sorus 

 constitutes a synangium, for the five sporangia 

 of which it consists are fused at the base, and 

 attached by a short common pedicel to the lower 

 surface of the pinnule (see Fig. 109, D). Hence their 

 arrangement is altogether that of a Marattiaceous 

 Fern. The structure, however, of the individual 

 sporangia is peculiar, for the free end of each is 

 capped by an apical annulus of large thick-walled 



1 Cours de botanique fossile, vol. iii. p. 122, 1883. M. Renault 

 named the species Pecopteris intermedia. Stur appropriately founded the 

 genus Renaultia for its reception, but this generic name was adopted 

 slightly earlier by Zeiller for another fructification, above mentioned. 



