106 THE POTOMAC OR YOUNGER MESOZOIC FLORA, 



apparently a nerve. The entire under surface of the pinnule is covered 

 by the sori. 



Locality: Fredericksburg. 



Only a small fragment of this curious fern was found, and conse- 

 quently little can be made out as to its true nature. It seems to be quite 

 different from all previously described fossils. 



ACROSTICHOPTERIS, gen. nov. 



Fronds probably creeping, with very long, often flexuons rachises, 

 which seem to have been more or less succulent; pinnte going off obliquely, 

 long and apparently -slender; ultimate pinnae or pinnules subopposite to 

 alternate, comparatively short, and cut down nearly to the rachis into more 

 or less cuneate-llabellate 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, 

 flabellatety diverging, forking dichotomously, and ending in the teetli; 

 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 close appressed to the 

 principal rachis. The fructified segments are so modified as to take the 

 form of leathery, rounded, or elliptical 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. 



This curious genus of ferns seems to be of a composite character 

 The sterile forms look so much like Bnicropsis that before the fructified 

 forms were found, which occur only at Baltimore, they were taken to be 

 small forms of that genus. 



The genus in the naked sori is like Polypnd'non, but in most features 

 stands nearest to Acrostichum, much resembling the section Bhip'idopteris. 

 In this latter, however, the fructification is borne on separate pinnules. If 

 we place the fructified pinnules of Rhipidopteris as basal segments on the 

 sterile ones, we have a form strikingly like Acrostkhopteris. This genus 

 has also some resemblance to Marsilea. 



