Maryland GtEological Survey 271 



15 feet in diameter as preserved and only 4 feet thick, the recognizable 

 remains practically all coming from the basal 3 to 5 inches. 



With the large amount of material at his disposal the writer finds it 

 altogether impossible to differentiate the forty species described by Pro- 

 fessor Fontaine from the Potomac Group. There are two main types, the 

 narrow pinnule type, that identified in some of the Baltimore specimens 

 as Sphenopieris Mantelli by Fontaine, and including some of the forms 

 described as new species of Thyrsopteris, and the broader type exemplified 

 by the foreign Sphenopieris Gcepperti. It is to the latter that a large 

 number of the Potomac forms belong. Three additional species, which 

 include the balance of the Thyrsopteris forms, are characterized. In 

 perusing the synonymy of the species which follow, the question is likely 

 to rise in the mind of the reader whether or not the process of ignoring 

 minor differences has not been carried too far, so that it is needful to 

 point out the reasons which have led to the present treatment. The main 

 reason is, of course, that it was found impossible to fix upon any char- 

 acters of specific value that would hold good for material other than the 

 individual specimen upon which they were based. That the author of 

 these species could not tell them one from the other is quite obvious in 

 looking over the material which passed through his hands, specimens 

 identical in all particulars at one time receiving one name and on a 

 subsequent occasion another, even counterparts of the same specimen 

 being, in at least one instance, identified as distinct species. 



These ferns were of large size with tripinnate fronds, so that it is 

 easy to see how one or two species with slight individual variations in 

 form could, when broken up into fragments and fossilized in a matrix 

 for the most part of very arenaceous clay, form the basis for numerous 

 species. The pinnae from the base of the frond will differ more or less 

 from those higher up and the basal pinnules of the individual pinnae 

 will differ decidedly from the distal ones. It is possible in the more 

 complete Potomac specimens to trace these variations and so get a num- 

 ber of Fontaine's types on a single specimen. It therefore seems wiser to 

 consider the bulk of the forms as exemplifying slight variations, due 

 largely to position, rather than to allow them specific or even varietal 

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