492 
SILURIAN TRILOBITES, 
P. Crossleii agrees very closely with P. rana, Hall, from 
which it differs in the number of lenses in, and position of the eye; 
and the absence in the latter of lateral furrows on the glabella. 
From our P. latigenalis it is separated by the greater propor- 
tional length and height of the eyes, and by the constant linear 
character of the glabella furrows, by the smaller space between 
the bottom of the eye and the cheek border, the more upright 
glabella cheeks, and wider axial furrows between the glabella and 
fixed cheeks. The glabella is also less expanded in front trans- 
versely. The pygidium has a more semicircular contour, and its 
axis differs from P. latigenalis by contracting more gradually 
from front to back, and in not being sunk between the side lobes. 
The head shields of young specimens of the two species do not 
show the differences in so marked a manner, nor are the thoraces 
of the two separable from each other in a decorticated state; but 
when the mature specimens of each species are compared the 
differences noted above are evident. 
Such a variation may be expected even were P. latigenalis the 
progenitor of this species, for they are separated by 3000 ft. to 
4000 ft. of strata, chiefly consisting of mudstone shales, which 
must represent a long geological period. 
From P. fecundus, McCoy, it is at once separated by the very 
much smaller eye, greater posterior contraction of the glabella, 
deeper and wider axial furrows of the cephalon, and by some 
differences in the pygidium. P. Crossleii is a smaller species than 
either of the others described in the present paper, or P. fecundus, 
Barr., none of our specimens exceeding two inches in length. 
It has been specifically named after Mr. R. Crossley, of White- 
field, Bowning, in recognition of much valuable assistance 
rendered by him to one of us in the collection of specimens. 
Loc. and Horizon. — Bowning Village, Co. Harden. Upper 
Trilobite Bed, Bowning Series ( = Hume Beds, Jenkins, and lass 
Beds, David)— ? Wenlock. Coll. Mitchell. 
