492 SILURIAN TRILOBITES, 



F. Crosshii 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. 



Log. and IIorizo7i. — Bowning Village, Co. Harden. Upper 

 Trilobite Bed, Bowning Series ( = IIume Beds, Jenkins, and I'ass 

 Beds, David)— ^ Wenlock. Coll. Mitchell. 



