328 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 41. 



Thyrsopteris crassmervrs Fontaine, Monogr. U. S. Geol. Surv., vol. 15, 1890, p. 130, 



pi. 41, figs. 1-3.— Fontaine, in Ward, 19th Ann. Rept. U. S. Geol. Surv., 



pt. 2, 1899, p. 658, pi. 161, figs. 3, 4.— Fontaine, in Ward, Monogr. U. S. 



Geol. Surv., vol. 48, 1906, pp. 513, 528, pi. 112, figs. 5, 6. 

 Thyrsopteris pecopteroides Fontaine, Monogr. U. S. Geol. Surv., vol. 15, 1890, 



p. 135, pi. 51, fig. 1.— Fontaine, 19th Ann. Rept. U. S. Geol. Surv., pt. 2, 



1899, p. 661, pi. 161, figs. 16-19. 

 Adiantites parvifolius Fontaine, in Ward, Monogr. U. S. Geol. Surv., vol. 48, 



1906, p. 558, pi. 117, fig. 1. 

 Thyrsopteris heteroloba Fontaine, Monogr. U. S. Geol. Surv., vol. 15, 1890, p. 139, 



pi. 53, fig. 4. 

 Thyrsopteris obtusiloba Fontaine, Monogr. U. S. Geol. Surv., vol. 15, 1890, p. 



143, pi. 58, figs. 7, 10. 



Description. — Frond bipinnate or tripinnate. Principal rachis 

 mediumly stout, somewhat flexuous, sometimes winged in the upper 

 part. Pinnae alternate or subopposite, ovate to ovate-lanceolate 

 in outline, becommg entire apicall}^, the pinnules passing into dentate 

 teeth while the ultimate pinnse become dentate pinnules. This char- 

 acter renders distal fragments quite different in appearance from the 

 normal form of this species and quite like CladoplileMs. In some 

 individuals the pinnse lower down on the frond assume this form, con- 

 stituting the supposed species Tliyrsopteris crassinervis of Professor 

 Fontaine and well showm in the specimens from Chinkapin Hol- 

 low and from near Glymont. Every gradation is shown, however, 

 between this type and the usual type of pinnae made up of alter- 

 nate,- very oblique, decurrent pinnules, usuall}^ rather deeply cut into 

 subrhombic basal lobes, which become ovate or elliptical lobes and 

 finally teeth in passing distad. Base contracted, subpetiolate. Veins 

 numerous and slender but very distinct, branching obliquely, flabellate, 

 repeatedly forked, subparallel. Texture coriaceous. 



Wliile the fragments of the fronds of this species are all small, the 

 plant which bore them must have been of considerable dimensions. 

 Representative material is readily distinguished from the other 

 species of Onycliiopsis recognized, but small fragments are liable to 

 confusion with Onycliiopsis hrevifolia; in fact, Professor Fontaine 

 founded no less than six nominal species upon such fragments, all of 

 which are beUeved by the ^\Titer to represent slight variations of a 

 single species. 



It is widely distributed throughout the Potomac group, but not 

 common at any outcrop. Outside this area it has been reported from 

 the Lakota formation of the Black Hills. Practically identical re- 

 mains from the Lower Cretaceous of Portugal are described by Saporta 

 as various species of Sphenopteris. 



Occurrence. — Patuxent formation: Fredericksburg, Dutch Gap, 

 Potomac Run, Virginia; New Reservoir, Ivy City, District of Colum- 

 bia; Sprhigfield, Maryland. Arundel formation: Langdon (fre- 

 quent), District of Columbia. Patapsco formation : Chinkapin Hoi- 



