324 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 41. 



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

 altogether impossible to differentiate the 40 species described by 

 Professor Fontaine from the Potomac group. There are two main 

 types, the narrow pinnule type, that identified m some of the Balti- 

 more specimens as SijlienoioUris mantelli by Fontaine and includmg 

 some of the forms described as new species of Thijrsopteris, and the 

 broader type exemplified by the foreign Splienopteris goepperti. 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 synonym of the species which 

 follow, the question is likely to arise m the mmd of the reader whether 

 or not the process of ignoring minor difterences has not been carried 

 too far, so that it is needfid to pomt 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 characters 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 m looldng over the 

 material which passed through his hands, specimens identical in all 

 particulars at one time receivmg one name and on a subsequent occa- 

 sion 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 m a matrix 

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

 species. The pinnsB from the base of the frond will difl^er more or less 

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

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

 complete Potomac specimens to trace these variations and so get a 

 number of Fontaine's types on a single specimen so that it 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 rank. The published drawings of these forms, especially the 

 enlarged pinnules showing detail, are for the most part inaccurate and 

 idealized to such an extent that even the experts in the National 

 Museum often find it impossible to decide which specimens represent 

 Professor Fontaine's drawings. 



With regard to our taking up the genus OnycJiiopsis of the Polypo- 

 diaceoe rather than Thyrsopteris of the Cyatheacese it may be said that 

 while Tliyrsopteris as a form-genus may not be open to any great 

 degree to criticism, it implies a relationship with the existing species 

 which the evidence does not substantiate so that the best modern 

 usage refers the older type of this sort to the genus Coniopteris Brong- 

 niart and the later ones to this genus OnycJiiopsis. It is quite possible 



