76 AMERICAN FERN JOURNAL 
Arizona and New Mexico. It is the purpose of this 
paper to point out in detail the differences between these 
three closely related species. 
Externally, the three species are fairly easy to dis- 
tinguish. P. glabella is the largest, the fronds measuring 
up to 25 em. in length (including the stipe) and, except 
in obviously depauperate plants, seldom measuring less 
than 12 em. In P. Suksdorfiana the average length of 
the frond is 9.5 cm. and it seldom exceeds 12 cm. In 
one of the Suksdorf specimens listed below the longest 
frond is 20 cm. long, but this is the only specimen seen 
which has any fronds over 15 cm. long. P. pumila is 
even more dwarf, the average length of the frond being 
6.8 cm., and the longest seen, 14 em. 
In P. glabella, except in very young or depauperate 
plants, the fronds are twice compound. The lowest 
pair of pinnae, which are the largest, are 3-5-foliolate, 
the second and third pairs are frequently compound also, 
and occasionally even pinnae higher up in the frond. 
The terminal pinnule of a compound pinna is stalked, 
the lateral pinnules are often over half of the length of 
the terminal one and closely resemble the small pinnae 
of the first order which occur towards the tip of the frond. 
Semi-compound pinnae occur frequently between the 
completely compound basal, and the simple upper 
pinnae. In these, the basal lobes, corresponding to the 
lateral pinnules of compound pinnae, are variously 
shaped, usually acute, and 14—14 the length of the cen- 
tral lobe. They are often asymmetrical, with the 
posterior basal lobe better developed than the anterior. 
Rarely one of the basal lobes is reduced to a mere auricle, 
and very rarely indeed both basal lobes are so reduced. 
P. Suksdorfiana shows much less tendency to form 
twice compound fronds than P. glabella of similar size. 
Frequently the pinnae are simple throughout, but 
occasionally the one or two lower pairs of pinnae are 3- 
