BY R. ETHBBIDOR, JUNR , AND JOHN MITCHELL. 491 
smooth border is exposed bounded on outer edge by a linear 
furrow. 
Obs. — This species in some respects resembles P. fecundus, 
Barr., with which we were inclined on first inspection to consider 
it to be identical. Closer examination, however, has revealed 
sufficient differences between them to justify us in separating it 
from that species. 
In the first place, in our species the greatest ■ length of the 
glabella, including the neck ring, equals its greatest width. In 
P. fecundus the glabella furrows are distinct on immature indi- 
viduals, but rarely so on mature ones, which is just the opposite 
to the case in our species. Again, the eyes of the latter have, so 
far as we have been able to observe in all our numerous speci- 
mens, a constant number of seventeen vertical rows of lenses, and 
never more than five lenses in a row, except in rare cases where 
a rudimentary lens occurs at the top. 
The lateral extension of the neck furrow around the edge of 
the border of the side lobes is not nearly so distinct in our species 
as in P. fecundus, Barr., and it ceases in front of the eye instead 
of joining the furrow passing round the frontal base of the 
glabella. 
Between the thoraces of the two species there appears little 
difference, except that the thoracic test of ours seems to have 
been smoother, and the sides more perpendicular. 
The pygidium of our species is more nearly semicircular, and 
its axis is not sunk between the side lobes, with an almost 
constant number of seven segments, and rarely if ever eight. The 
side lobes are divided into six or, doubtfully, seven pleurae. 
To sum up, our species is separated from P. fecundus by having 
a much smaller eye, the features of which remain constant in all 
mature individuals, a less distinct furrow separating the glabella 
in front from the rudimentary limb, by a smaller number of 
divisions in the axis and side lobes of the pygidium; and 
apparently a much thinner test, which was less distinctly granu- 
lated on the thorax and pygidium. 
