Makyland Geological Survey 301 



Occurrence. — Patuxent Formation. Fredericksburg, near Potomac 

 Eun, Dutch Gap, Kankeys, Chinkapin Hollow, near Telegraph Station 

 (Lorton), and Cockpit Point (?), Virginia. 



Collection. — U. S. National Museum. 



Genus THINNFELDIA Ettingsliausen 

 [Abh. k. k. geol. Reichsanstalt, Band i, Abth. iii, No. 3, 1852, p. 2] 



The genus Thinnfeldia was proposed by Ettingshausen in 1852 with 

 Thinnfeldia rhomb oidalis as the type, for remains from the Lias of 

 Steierdorf in Hungary. He characterized the genus in the following 

 terms: 



" Rami teretes vel subalati. Folia disticha, alterna oppositave, rhom- 

 boidea, ovalia vel lanceolata vel linearia, flabellatim vel pinnatdm 

 venosa." 



After comparing his species, three in number, with Odontopteris, 

 Pachypteris, Alhertia, Dammara, Qinhgo, and PTiyllocladus, Ettings- 

 hausen was induced to place Thinnfeldia Avith the conifers, from its 

 resemblance to the existing Phyllocladus. In 1867 Schenk, on the basis 

 of material from the Rhsetic of Franconia, which showed epidermal and 

 stomatal characters, referred Thinnfeldia to the cycads,^ while Schimper, 

 Saporta, Seward, and others refer it to the Filicales. 



Raciborski ^ has furnished good eAddence for the belief that some 

 at least of the material from the European Jurassic is filicinean, as 

 seems also to be the nature of the Potomac material, while the writer 

 has shown " that the later American species which have been referred to 

 this genus are in all probability gymnospermous. 



Various authors have called attention to the difficulty of distinguishing 

 Thinnfeldia from such genera as Pachypteris Brongniart Dichopteris 

 Zigno, Cycadopteris Zigno, Lomatopteris Schimper, and while the con- 



^ Schenk, Die fossile Flora der GrenzschicMen der Keupers und Lias 

 Frankens, 1867, p. 105, pi. xxvii. 



'■' Raciborski, Fl. Kopalna ogn. Glinek Krakowskich, 1894, see pi. xx, figs. 

 1, 2. 



^ Berry, Bull. Torrey Club, vol. xxx, 1903, pp. 438-445. 



