124 



NATURAL HISTORY BULLETIN. 



CyathecB, on the other. The Osmundacece, by the structure 

 and disposition of the fibro-vascular systems in the stem, 

 represent the highest development of the sub-class, standing 

 in this respect close to the Coniferoz. 



The arrangement of the primary groups in this paper is 

 that of Goebel's " Outlines of Classification." 



The key which follows represents a grouping of forms 

 according to the characters already cited. 



In the sub-order Polypodiacece it is necessary again to note 

 that an arrangement in a linear series is not possible. In the 

 list the Families are of course placed in consecutive order, but 

 the series here again is branching. 



This sub-order illustrates development in two directions. 

 First from the indusiate and involucrate type presented by the 

 Pteridece along two branches, one of which culminates in the 

 JJicksoniece and the other in the Aspidiece, and second in the 

 group of non-indusiate forms beginning with the Acrostichece 

 with their scattered sporangia, and culminating in the Poly- 

 podiece with well-defined sori. 



The Families of each of the divisions so marked, are ar- 

 ranged with reference to the extent of the sori, those having 

 the sori most limited in extent being placed highest. 



Consequently in the consecutive arrangement which is 

 necessary in the list, families like the Dicksoniece, As-pidiece, 

 and Polypodiece, which are really closely related, are seem- 

 ingly widely separated. 



The arrangement which is presented in the following dia- 

 gram shows the relation more clearly. 



Sub-order, Polypodiacece. 



/ 



I. Pteridece. 



II. Lindsayece. V. Blechnece. IX. Acrostichece. 



III. DavalliecB. VI. Asple?iiece. X. Grammitidece. 



IV. Dicksoniece. VII. Cyathece. XL Vittariecz. 



VIII. Aspidece.— XII. Polypodiece. 



