154 THE POTOMAC OR YOUNGER MESOZOIC FLORA. 



forward; leaf-substauce thick, margins entire or with spinous teeth; nerves 

 several, arising from the racliis either separately, or from a common point, 

 or from a short parent nerve, branching one or more times, branches turned 

 slightly outward, or with a central nerve that sends off on each side 

 branches that diverge flabellately. 



The forms from the Potomac flora that I have placed in the genus 

 Ctempteris, with the amended description given above, differ somewhat from 

 the typical form on which Brongniart founded the genus, viz, Ctenopteris 

 cycadea Brong., and the species of Saporta, C. rjrandis, but stand very 

 near to the latter. 



These Potomac species apparently are composite in type and unite 

 some of the features proper to several genera of ferns. They have, how- 

 ever, so many of the characters of Ctenopteris that it seems proper to place 

 them in that genus. 



Perhaps the differences shown when they are compared with the 

 typical ctenopterids are due to modifications produced in the genus as time 

 passed, for we must remember that Ctenopteris cycadea and C. grandis are 

 Liassic fossils, while the Potomac forms are much } ounger. The diverg- 

 ences are of such a nature as to suggest this relationship, for the chief 

 ones consist of a greater complexity produced by the differentiation of the 

 nerves, which tends to produce a midnerve, and by the develoi)ment of 

 teeth on the margins of the pinnules or segments 



Although from the general character of these plants I regard them as 

 ferns, still, so long as they show no fructification, it is quite possible that 

 they may be cycads. Schimper^ well says that the tripinnate character of 

 the leaf of the genus does not necessarily exclude it from the cycads, for 

 the cycad genus Bowenia has bipinnate leaves. Bowenia resembles the 

 Potomac plants more strongly than the forms on which Brongniart founded 

 the genus, for this cycad has leaflets cut away obliquely in front and 

 nerves which diverge from a common point at the base of the piiuiules, 

 features found in the Potomac plants; and, what is more significant, some 

 of the pinnules of Bowenia have at least one spinous tooth, much like 

 those of the Potomac plants. 



' Zittel's Haiirlbuch der PaUooutol., vol. 2, Liferung I, p. 123. 



