132 THE POTOMAC OR YOUNGER MESOZOIC FLORA. 



compound pinna ; ultimate pinntie of the upper part of the frond (see Fig. 

 2) reduced to acute, elongate, elliptical lobes and teeth; the ultimate pinnae 

 subopposite to alternate, obliquely placed, linear-lanceolate, subacute, with 

 slender midribs terminated by an elliptical subacute lobe, attached to the 

 principal i-achis by a much narrowed base, composed of the midnerve with 

 a nari-OAV wing ; pinnules small, linear-oblong to elongate-elliptical, acute 

 to obtuse, basal ones separate nearly to the midnerve, those of tlie ultimate 

 pinna? slightly notched into acute teeth; pinnules towards the tip of the 

 ultimate pinnaj and higher on the frond without the teeth and more united 

 at base; nerves in the basal pinnules composed each of a slender midnerve, 

 which sends off on each side alternate simple nerves which go one to each 

 tooth ; in the lobes and simple pinnules higher on the frond, the nerves once 

 or twice forked or simple ; usuall}' the second forking occurs in the upper 

 branch of the forking nerves and is found in the lower pinnules of the frond 

 on the ultimate pinna-. 



Localities : Fredericksburg, not uncommon ; hill-side near Potomac 

 Run; rare. 



Tliis fine plant belongs to the Sphenopteris Mantelli type of Thyrsop- 

 tcris, and is in some of its forms like T. rarinervis and T. aiigitstifolia. It 

 is not winged, however. All the specimens figured except the small one 

 (Fig. 5) come from Fredericksburg. 



Thyksopteris pachyrachis, sp. nov. 



Plate XLVI. Figs. 3, 5; Plate XLVII, Figs. 1,2; Plate XLIX, Fig. 1. 



Frond tripinnate, large, arborescent; pinnse and pinnules usually 

 crowded; rachises ver}' stout, round, and prominent; ultimate pinnae op- 

 posite to alternate, lanceolate in outline, rather short; pinnules passing 

 rapidly into lobes and teeth towards the upper part of the frond and 

 compound pinna? ; lower piiuiules narrowed to the base, oblong or sub- 

 quadrate, acute, cut more or less deeply and very obli(piely into narrowly 

 elliptic acute teeth. In the upper parts the pinnules become narrowly 

 elliptic, toothed, and united by decui-rent bases, or pass into very oblitpiely 

 placed teeth of the same shape. The ultimate pinna? pass into pinnules 



