Maryland Geological Survey 331 



longitudinal; its inner walls showing the longitudinal rows of the alter- 

 nating ovate scales of the medullary rays. 



Genus DIOONITES Miquel 



[Over de Rangschikking der fossiele cycadese; Tijdschr. v. d. Wissen. Naturk. 

 Wetensch., Deel iv, 1851, p. 211] 



The question of the proper generic reference of cycad fronds is a much 

 mooted one, and considerable has been written as to the proper generic 

 name for the fossils here referred to Dioonites. Nathorst {loc. cit., p. 46) 

 and Seward {loc. cit., p. 75) have been foremost to question this refer- 

 ence, the former proposing the genus Zamiophyllum for fronds of this 

 type from the ISTeocomian of Japan because the pinnge are slightly nar- 

 rowed towards the base and the latter advocating their reference to 

 Zamites. Prof. Fontaine (1906, p. 246 et seq.) certainly makes out a 

 strong case for their retention in Miquel's genus which was characterized 

 as follows : " Frondes piunatse, rigidae, crasste. Foliola densa pantentis- 

 sima suprema nunc subimbricata, lanceolata vel lineari-lanceolata, recta 

 vel subfalcata, acuta vel acutiuscula, basi tola latitudine inserta, inferne 

 retroTsum subdecurrentia, nervis cum margine parallelis sequalibus sub- 

 tus, distinctioribus, cum sulculis stomatiferis alternantibus.'^ 



Bornemann in 1856 makes use of Miquel's genus, referring to it sev- 

 eral additional species including the one so common in the Potomac 

 Group, which Ettingshausen had previously referred to Pterophyllum. 

 Schimper (1870) also adopts it and gives a somewhat unintelligible 

 Latin diagnosis. The genus is also used by Schenk and Saporta. Fon- 

 taine in 1890, misled by Schimper's " pro- et decurrentibus," gives a 

 rather faulty generic diagnosis which is corrected in his later work 

 (1906). 



Dioonites may be distinguished from Pterophyllum by the insertion 

 of the pinnae on the plane of the upper surface of the rachis, and from 

 Zamites by the decurrent pinnas, for while they are slightly narrowed 

 toward the base in some specimens they are attached by their whole width 

 and run somewhat over and down the rachis, their insertion being some- 

 what lateral on the upper face of the rachis, the upper epidermis of the 



