288 MESOZOK' FLOKAS OF UNITED STATES. 



The plant lo which tlu'se forms l)elong was ovideutly a ivrn of .small 

 size and apparently- hei'haeeous in hal)it. It seems to have had finely 

 cut sterile leaves, of tliin hut Hrm and dural)le textvu'e. The structure of 

 the sori could not with positiveness be made out in detail, hut agrees l)e&t 

 with Dicksonia among living ferns. The different forms agree best with 

 the supposition that the involucre was bivalved, with the sporangia 

 sessile and covering the inner surface of the sorus. While the l)i\'alve 

 nature of the involucre is not certainly shown the sori are evidently 

 large, single, orl)icular to reniform, and l)orne at the summit of a ner\-e 

 included in a tliickened and much narrowed lobe of the pinna. The 

 form given in PI. LXXI, Fig. ;^. hears a striking likeness to Thyrsopteris, 

 l)ut that given in Fig. 1 shows that, unlike Thyrsopteris, the sut:)divisions 

 of the pinna are not wholly metamorphosed, but still retain something 

 of their foliaceous character. In Dicksonia the lacinife are narrowed 

 and thickened, it is true, l)ut not nearly so nmcli so as in this plant. I 

 have with hesitation placed this Montana fossil with this latter genus. 

 It is very near the Jurassic plant Heer has described as Dicksonia davipes," 

 but is obviously a different species. It is possible that l)oth of these 

 plants are not true Dicksonia, but a new genus intermediate between 

 Dicksonia and Thyrsopteris. It is highly probable that in this early 

 period there were such connecting links between these two genera that 

 are so near together. 



DicKsoxiA I'ACHYPHYLLA Foiitalne.'' 



PI. LXXI, Figs. 5-11. 



Several small and imperfect specimens of a fern were found that 

 seem to be a Dicksonia different from D. montanensis. Several of them 

 are fruiting, and one is a portion of a sterile pinnule (PI. LXXI, Fig. 5). 

 I am not sure that this form belongs to the same plant with that show- 

 ing the specimens in fruit, and am equally in doul^t whether or not the 

 fmiting forms belong together. All of them, however, have a similar 

 facies and have characters in common that justify placing them provi- 

 sionally in the same species until more and better specimens are obtained. 

 All of them liave a i-ather broad, flat niidril), with strong lateral nerves 



"FI. Foss. Arct., Vol. JX, Pt. II: Beitrage zur Jura-Fl. Ostsibiriens und d. Amurlaiules, p. 33, pi. ii, 

 fig. 7. 



(- See p. 224. 



