460 SCIENCE PROGRESS. 



palaeobotanical writings are those relating to fern fronds ; a 

 mere external resemblance in form between an imperfect 

 specimen of a fossil frond and the leaf of a recent species has 

 often proved too strong a temptation for even an experienced 

 specialist. Many of the Mesozoic ferns referred to Thyrso- 

 pteris — a genus represented to-day by a solitary species 

 confined to Juan Fernandez — Aspidium, Hy7nenopkyllum, 

 Dicksonia and other genera have no claim whatever to be 

 accepted as well-established representatives of these generic 

 types. 



One of the commonest and most characteristic Wealden 

 ferns is that named by Brongniart in 1828 Sphenopteris 

 M ante Hi ; the tripinnate fronds of this plant are ovate- 

 lanceolate in form, with narrow uni-nerved ultimate seg- 

 ments. The fertile pinnae bear ovate and somewhat swollen 

 segments, which present a striking agreement with those of 

 some species of the recent genus Onychium. Although no 

 individual sporangia have been preserved, the form of the 

 fertile segments — in addition to the general similarity of 

 habit — points to a close affinity with some of the tropical 

 species of Onychium. On the ground of this resemblance, 

 the generic name Onychiopsis has been substituted for that 

 of Sphenopteris. 



Casts of a Tree-fern stem usually referred to the genus 

 Protopteris, but spoken of by some writers as Dicksonia, 

 are not uncommon in rocks of Lower Cretaceous age ; the 

 stem is covered with crowded spirally disposed petiole- 

 bases, in each of which the form of a horseshoe-shaped 

 leaf-trace is clearly shown, and in an English example the 

 vascular tissues of the stem have been partially petrified. 

 In form and structure this genus presents a striking agree- 

 ment with the recent Tree-fern Dicksonia antarctica Lab., 

 and it is highly probable that this species is closely related 

 to the Wealden plant. Some fronds of Wealden age have 

 also been referred to Dicksonia, and these lend a certain 

 amount of support to the comparison which has been made 

 of the Protopteris stems with those of the recent genus. 



A Wealden fern originally described by Dunker * from 



1 Dunker, p. 5, pi. ii., fig. 2. 



