288 MESOZOIC FLORAS OF UNITED STATES. 



The plant to which these forms belong was evidently a fern of small 

 size and apparently herbaceous in habit. It seems to have had finely 

 cut sterile leaves, of thin but firm and durable texture. The structure of 

 the sori could not with positiveness be made out in detail, but agrees best 

 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 bivalve 

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

 large, single, orbicular to reniform, and borne at the summit of a nerve 

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

 form given in PI. LXXI, Fig. 3, bears a striking likeness to Thyrsopteris, 

 but that given in Fig. 1 shows that, unlike Thyrsopteris, the subdivisions 

 of the pinna are not whoU}'^ metamorphosed, but still retain something 

 of their foliaceous character. In Dicksonia the lacinia^ are narrowed 

 and thickened, it is true, but not nearly so much 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 clavipes," 

 but is ol^viously a different species. It is possible that both 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. 



Dicksonia pachyphylla Fontaine.^ 



PL LXXI, Figs. 5-n. 



Several small and imperfect specimens of a fern were found that 

 seem to l^e a Dicksonia different from D. montanensis. Several of them 

 are fruiting, and one is a portion of a sterile pinnule (PL 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 doubt 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 have a rather broad, fiat midrib, with strong lateral nerves 



«F1. Foss. Arct., Vol. TV, Ft. II: Beitriige zur Jura-Fl. Ostsibiriens und d. Amurlandes, p. 33, pi. ii, 

 fig. 7. 



b See p. 224. 



