96 THE POTOMAC OR YOUNGER MESOZOIC FLORA. 



Only the fructified form of this fern was seen. The dehiscence of 

 the sori seems to be marg-inal, and there are indications of the escaping 

 sporangia, as shown in PI. XIII, Fig. 9", which represents a greatly 

 enlarged sorus. Tlie plant seems to be a distinct new species. 



ASPIDIUM HETEROPHYLLUM, Sp. nOV. 

 Plate XIV, Figs. 1-5 ; Plate XV, Figs. 1-5. 



Frond tripinnate, arborescent; rachises of the pinnae of various orders, 

 strong and rigid; principal pinnae veiy large, spreading widely; penulti- 

 mate pinnae alternate, in outline elongate-oblong or elliptical, acuminate, 

 abruptly and greatly narrowed towards the summit, slightly so towards the 

 base ; ultimate pinnae linear, acute, quite short, being longest towards 

 the middle of the penultimate pinna^ at first rapidly shortened towards the 

 upper part and then gradually giving the acuminate terminations of 

 the penultimate pinna\ The basal ultimate pinna on the lower side of the 

 penultimate pinnas is usually abnormally short and broad, being inserted 

 in the angle made by the junction of the penultimate pinna with the prin- 

 cipal rachis. 



Towards the summit of the penultimate pinna? the ultimate ones pass 

 through pinna? reduced to toothed pinnules into those which finally 

 become simple pinnules; pinnules varying according to position on the 

 frond, usually alternate, short, acute to obtuse, slightly falcate ovate, 

 attached by a widened base; those from the lower part of the frond with 

 dentate or undulate margins, those from the middle portions with entire 

 margins, those from the summit of the pinnae of tlie various orders united 

 to form teeth. The basal pinnule on the lower side of the ultimate pinnae, 

 inserted in the angle between the ultimate and penultimate rachises and 

 partly on the latter, dift'ers from tlie normal pinnules in shape and size, 

 being heteromorphous and larger than the normal one. It is furnished with 

 a lobe or tooth on the losver side. In the sterile forms the character of the 

 pinnules and of the nervation is that of Cladophlebis. The pinnules have 

 each a midnerve sending off alternately on each side slender but distinct 

 lateral nerves, simple or once forked. Fructified forms have the pinnules 

 mostly obtuse, while those in the sterile portions are acute; the sori are 



