120 THE POTOMAC OK YOUNGER MESOZOIO FLORA. 



going off obliquely or under an acute angle ; fertile pinna3 contracted ; 

 sori globose, paniculate or racemose, inserted on a pediceled involucre. 



This description, given by Heer^ for the genus Thyrsopteris, so far as 

 the portion pertaining to the sterile frond is concerned, agrees well witli a 

 large number of species in the Potomac flora. These I place provisionally 

 in the genus Thyrsopteris, on account of the great resemblance that tlie 

 shape of the pinnules, the lobing, and the nervation show to the sterile 

 forms of various species determined to be Thyrsopteris by their fructifica- 

 tion. As, however, no fructification is found in the Potomac species, the 

 placing of these plants in the genus must be regarded as provisional. It is 

 quite possible that some of them belong to Aspidinm and Dicksonia. The 

 genus Thyrsopteris seems to be eminently a Jurassic type of fern. How- 

 ever it may prove to be with the fructification of tlie various species 

 assigned in this memoir to this genus, it is probable that the)' are at least 

 distinct and true species. Tlie great variety and comparative abundance 

 of these forms show that this type was developed in the Potomac flora to 

 an unusual degree. They surpass all other types, even that of Cladophle- 

 bis, and give a decided Jurassic facies to the assemblage of ferns found 

 in this flora. Even if we can not by fructification determine the affinities 

 of fossils with living plants, the large development of sterile forms of well- 

 marked types is of importance in fixing the character of a flora. 



It should be noted that a number of the species of Thyrsopteris 

 described in the following pages show a good many features similar to 

 those of Sphenopteris Mantelli, as described by Schenk and Heer. 



Thyrsopteris Virginica, sp nov. 



Phate XXIV, Fig. 1. 



Frond not seen; j^innjc of ultimate order, linear-lanceolate, acute, nar- 

 rowed to a petioled base, and cut very obliquely into oblong lobes, the 

 lower of which are obtusely dentate; racliis of ultimate pinnules very 

 slender ; lateral nerves in each lobe flabellate, sending off" branches which 

 in the lower lobes are once or twice divided, in the upper ones forked or 

 simple, rather remote, veiy distinct and strong. 



' Flor. Foss. Arc, vol. 4, pt. 2, p. 28. 



