Maryland Geological Survey 351 



costa exorientes simplices furcatique divergentes, nervo medio nullo, 

 nervulis mediis dense quandoque f asciculatis ; fructificatio ignota." 



The Potomac species are bi- or tripinnate with broadly linear-lanceo- 

 late pinnae of thick texture, with entire or toothed margins. The 

 botanical position of this genus has never been definitely settled. Saporta 

 has called attention to its resemblance to the Odontopteris forms of the 

 Paleozoic and to the Oolitic genus Dichopieris of Zigno. Schimper 

 makes it the basis for his filicinean genus Cycadopteris. ISTathorst pro- 

 posed the term Ctenozamites for remains of this sort and Seward origi- 

 nally referred the English Oolitic material to Ptilozamites. While 

 Fontaine regarded the Potomac species as ferns it seems very probable in 

 view of the general habit of the fronds and in the absence of the fructi- 

 fication characters that they are fronds of cycadophytes and they are so 

 considered in the present work. As has been repeatedly pointed out, the 

 modern cycad genus Bowenia furnishes an analogy among recent cycads 

 and when we recall the probable diversity of the Mesozoic cycadophytes 

 and their close relationship with the ferns, and particularly their filiation * 

 with forms with the fern-like foliage of the Paleozoic Pteridospermato- 

 phytes^ to which group at least some of the Odontopterids belong, there 

 can be little objection raised to such a reference on the basis of frond 

 characters. 



The genus Ctenopteris is mainly Jurassic, originating as far as the 

 present records show in the Lias, and being especially well developed in 

 the Oolites. Besides the three Potomac species to which the writer has 

 reduced the six species of Fontaine, Saporta has described a ISTeocomian 

 species from the Portuguese rocks and Penhallow another from the 

 Cretaceous of Vancouver Island. In addition we have the closely allied 

 cycadaceous genus Ctenidium Heer with two species in the Neocomian 

 and Cenomanian of Portugal. 



The Potomac species, which in many respects suggest the genus 

 Zamiopsis of Fontaine, are confined to the Patuxent and Arundel for- 

 mations and are abundant in the older deposits at Fredericksburg, 

 Virginia. 

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