92 THE POTOMAC OE YOUNGEK MESOZOIO FLORA. 



This is Heer's' description. To this he adds that the upper pinnules 

 have only a midnerve, while the lower ones have from one to three secon- 

 dary nerves, which detach themselves at a very acute angle. 



The species S. MantelU seems to be quite polymorphous, and different 

 authorities on fossil plants have united with this species a large number of 

 plants which vary considerably and come from widely separated regions. 

 There is quite a large number of closely allied plants in the Potomac 

 flora which have moi-e or less of the characters of S. MantelU. These may 

 be noted as belonging to the type of this species, but it does not seem 

 advisable to combine them under one species. Most of tliem have the 

 characters of Thyrsopferis, and I have described them as belonging to that 

 genus. It is a fact worthy of mention that there are in the Potomac flora 

 several types which resemble described fossils, and under which a number 

 of allied species may be grouped. Thus we find the type of Splienopteris 

 MantelU, of Neiiropteris Alhertsu Dunkr., of Fecopteris Broioniana, of Cla- 

 doplilehls {Pecopteris) Whitbiensis, etc. 



In using the name Splienopteris MantelU, I have restricted its applica- 

 tion to the forms given on PI. L, Figs. 1, 2, which have clearly the char- 

 acter of the original species. These specimens come from Baltimore, and 

 are found among the specimens collected by Meek at that place. These 

 specimens do not have their locality fixed exactly, but it appears probable 

 that they were obtained from Fe deral Hill near or on Covington street. 

 The fragments supposed to be S. MantelU have the principal rachis com- 

 paratively very stout, with alternate pinnules, which are cut into narrow 

 linear incurved lobes and teeth, each having a simple slender nerve. 

 Among Meek's specimens there is one form given in Fig. 2, wliich is 

 smaller than the normal one represented in Fig. 1. In this smaller form 

 the principal pinnaj are opposite, and the lobes of the pinnules much more 

 minute than in the fossil depicted in Fig 1, Probably this plant, with the 

 more slender pinnules and lobes, simply represents a different part of the 

 frond from that given in Fig. 1, and hence the difference in dimensions. 

 The plant is evidently rare at Baltimore, at least on the horizon of the beds 

 yielding the specimens found on Belt and Covington streets, for no spec- 



' Flor. Fos3. dii Portugal, p. 12. 



