60 ACROSTICHOPTERIS. 



Cf. Thyrsopteris Jlfeekiana, var. cwgustiloba, Font., pi. Ivi. etc. ; 

 also Sphenopteris Mantelli, Schenk, Palaeontographica, vol. xix. 

 pi. xxv. fig. 6 ; Sphenopteris Gopperti, Schenk, loo. cit. pi. xxx. 

 fig. 2 ; and Asplenium Dicksonianum, Heer, from the Kome beds 

 of Greenland, PI. foss. Arct. iii. pi. i. Ecclesbourne. 



Ru/ord Coll. 



Genus ACROSTICHOPTERIS, Fontaine. 

 [Potomac Flora, U.S. Geol. Surv. Monograph, xv. 1889, p. 106.] 



Fontaine has instituted this genus for certain fossil ferns 

 " peculiar to the Potomac formation" ; he considers that it stands 

 Dearest in most features to Acrostichum among recent genera. 



The genus is thus described 1 : 



"Fronds, probably creeping, with very long, often flexuous, 

 rachises, which seem to have been more or less succulent ; pinnae 

 going off obliquely, long and apparently slender; ultimate pinnae 

 or pinnules sub-opposite to alternate, comparatively short, and 

 cut down nearly to the rachis into more or less cuneate-flabellate 

 pinnules or primary segments. These are divided generally into 

 cuneate-flabellate segments, which in turn are separated into 

 oblong segments ending in oblong, or ovate-obtuse, or acute 

 teeth ; pinnules decurrent and forming a wing ; nerves slender 

 but distinct, flabellately diverging, forking dichotomously, and 

 ending in the teeth ; fructification occurring on the basal segments 

 of the pinnules, in the upper portions of the frond on the upper 

 one alone, in the lower portions on the upper and lower ones; 

 the fructified segments, which on the lower side are covered by 

 the naked sori, and seen from the upper side, especially when 

 compressed on the clay, look like pods." 



In the figures of the Potomac species of this genus there are 

 several fertile specimens shown, but no detailed sporangial structure. 

 Perhaps the best figure is that of a fertile pinnule of Acrosti- 

 chopteris longipennis, Font., pi. clxxi. fig. la. Fontaine concludes 

 that " the genus in the naked sori is like Poly podium, but in 



1 Potomac Flora, p. 106. 



