Maryland Geological Survey 363 



Description. — Fronds large, upwards of 50 cm. to 70 cm. in length by 

 15 cm. to 18 cm. in greatest width, averaging about 13 cm., either 

 entire or more often irregularly divided into rectangular or subrhombic 

 segments, at times somewhat rounded basally. Texture coriaceous. 

 Eachis stout, prominent below, more or less flattened during fossiliza- 

 tion. Laterals of small calibre, close, -J to 1 mm. apart, parallel, in- 

 variably simple and nearly straight, diverging at an angle in the neigh- 

 borhood of 90°. 



This splendid species, presumably because of its large size, is repre- 

 sented only by fragmentary specimens, both the apex and the base being 

 missing. It is not at all common and is confined entirely to the Patuxent 

 and Arundel formations in the Maryland- Virginia area. The specimens 

 show some individual variations in the degree of segmentation, and 



b 



Fig. 11. — Cross-sections of fronds of Nilsonia densinerve. a — showing 

 method of fossilization of specimen shown on pi. Ivii, fig. 1, natural size. 

 b — Diagrammatic cross-section of specimen shown on pi. Iviii, fig. 1, natural 

 size. 



strength and position of the lateral veins, the forms which served as a 

 basis for Platy pterygium liogersianum of Fontaine being more robust 

 but somewhat smaller than the others with veins somewhat larger in 

 size and less closely placed, but the limits of variation are, nevertheless, 

 quite restricted. 



There can be but little doubt of their all belonging to a single species. 

 The accompanjdng plates, which illustrate in an admirable manner the 

 variation in appearance and manner of preservation of these fronds, are 

 photographs of three of Fontaine's figured specimens, which ones it will 

 rest with the reader to discover among the figures cited (supra), the 

 writer being wholly unable to determine. At first glance, the rachis 



