542 MESOZOIC FL()1{AS OF UNITED STATES. 



pinna, or a lohed pinnule. This is represented in Pi. CXIV, Fig. 8. 

 Fig. 9 gives two such fragments not so complete as that given in Fig. 8. 

 These two, however, are so placed as to indicate that they were once 

 attached to a common rachis, not now preserved. The pinnae or lolled 

 pinnules partly overlap. The parts that are preserved appear to be the 

 terminal ones of the pinna or pinnules and they are not sufficiently well 

 preserved to give their dimensions and shape. The nature of the incisions 

 of the lamina, which represent either loljes or pinnules, can be made out 

 and the nervation is remarkably distinct. This specimen is without 

 number or locality label, but the Arlington material is so distinctive that 

 there can be no doubt that it is from that locality. 



The lobes or pinnules are very obliquely incised and are ol^long in 

 form, with the free ends obtu.se lancet shaped. The incisions visible are 

 not cut down to the midril), but indicate that lower down on the portions 

 shown they may be so, constituting pinnules. The midnerve or rachis 

 of the pinna is distinct and somewhat flexuous. On each side of this 

 midnerve parent nerves depart at a very small angle to enter the pinnse 

 or lobes. The parent nerve forks at long intervals, the principal branch 

 of each fork keeping near the middle of the j)inna or lobe, while the other 

 branch forks some distance up. These minor nerves are tjuite remote 

 from one another and sharply defined, though not very strong. 



While this fern can not be fully made out, it is clearly different 

 from any previously found in the Potomac beds. It is confined to 

 the Arlington localities. 



AGE OF THE ARLINGTON BEDS. 



As was stated in the beginning of the account of the Arlington beds, 

 the plants occur at different localities in a similar kind of rock and are gen- 

 erally the same, so that the presumption is that the fossiliferous beds at 

 all of them are essentially the same. To determine their age, as com- 

 pared with the divisions of the Virginia Lower Potomac, comparison 

 must be made with the plants described in Monograph XV. The lal)els 

 accompanying most of the fossils give the Maryland division as Patapsco. 

 Those from one of the localities are given as Arundel. 



