82 THE POTOMAC OR you:nger mesozoic flora. 



of the miduerve in a jjiiinate manner, and at a vaiying angle, simple, or 

 several times forked, not anastomosing. 



The genvis Pecopteris, based mainly on the nervation and probably 

 containing many species of distinct ferns, serves simply as a convenient 

 grouping of plants having a particular facies, and whose generic character 

 can not be otherwise fixed. There are a number of forms in the Potomac 

 flora which have the character given above for Pecopteris, and which is 

 taken from Schimper's description of the genus. The number of ferns, 

 however, found in the Potomac flora which could be placed in the genus 

 Pecopteris is far less than that of those which possess the characters of 

 CladopJileb'ts and Ihyrsoptcris. This type is rather feebly represented in the 

 Potomac flora. 



Pecopteris Virginiensis, sp. nov. 



Plate VIII, Figs. 1-7 ; Plate IX, Figs. 1-6 ; Plate XXIV, Fig. 2; Plato CLXIX, Fig. 3. 



Frond bipinnate or tripinnate, arborescent; pinnae alternate, very 

 long ; leaf-substance thick and durable ; tlie principal racliis and that of 

 the ultimate pinnre stout, rigid, with the margins on the upper face raised in 

 the form of a cord ; pinnules elongate-oblong to narrowly linear, opposite 

 to alternate, much narrowed at the base and separate to the base, cut 

 away obliquely on the upper side and decurrent slightly on the lower one, 

 or else united at base to form a wing along the rachis of the pinna?; mar- 

 gins denticulate to distinctly dentate, straight or more commonly slightly 

 falcate ; midnerve prolonged to near the tips of the pinnules, often rather 

 slender but distinct; lateral nerves once forked, one of the branches 

 usually ending in the tip of the tooth on the margin. 



Localities : Red clay ball in the banks of Dutch Gap Canal ; fishing 

 hut above Dutch Gap Canal; Fredericksburg; road-side near Potomac 

 Run ; 72d mile-post, near Brooke ; near Telegraph Station ; Covington 

 street, Baltimore. 



This is perhaps the most widely diffused of the Potomac plants. At 

 the locality on the road-side near Potomac Run the fragments are among 

 the most common. They occur in such a way in the irregularl}' deposited 

 clay as to suggest that the dimensions attained by this fern were great. 

 The clay was of such a»nature at this locality as to render it impossible 



