Yol. 52.] FAUNA OF THE KEISLEY LIMESTONE. 421 



his work on the East Baltic trilobites. Some of our specimens do not 

 correspond with the type-form of the species, but more closely with 

 the variety which Schmidt describes (op. cit. p. 176) ; and the very 

 close relationship of the two species Ps. hemicranium (Kut.) and 

 Ps. conformis (Ang.) var. of Schmidt is brought out by a minute 

 comparison of them. Schmidt himself says that these two species of 

 PseudosplKxrexochus stand very close to each other, and the feature 

 of the greater length of the glabella is almost lost in the variety 

 from Borkholm. 



In addition to several head-shields, a typical pygidium has been 

 found at Keisley which measures nearly 2 millim. in length (ex- 

 cluding the spines), and 3*5 millim. in breadth. 



It was from a pygidium similar in all respects to this one that 

 Harkness recorded Ch. octolobatus from the Keisley Limestone. 



Cheirttrus (Psetjdosph^erexochus) subqtjadratus, sp. n. (PI. XX. 

 figs. 10 & 11.) 



In addition to Ch. (Ps.) conformis (Ang.) in the Keisley Limestone, 

 there occurs in greater abundance a form which must be considered 

 very closely allied to Kutorga's species Ch. (Ps.) hemicranium, which 

 has been described by various authors, and recently with minuteness 

 by Schmidt. 1 The characters of the Keisley form are well marked 

 and constant, but unfortunately only the head-shields are known. I 

 hesitated for some time whether it could be satisfactorily separated 

 from Ch. (Ps.) hemicranium, but with the advantage of more 

 material for examination I am convinced that the new species rests 

 on quite as firm a foundation as Ch. (Ps.) conformis (Ang.). 

 The description is as follows : — 



The head-shield is almost semicircular in form, but the length is 

 a little more than half the breadth. The glabella occupies more 

 than one-third of its breadth and is rounded-subquadrate in shape ; it 

 is strongly convex and much elevated above the downwardly-sloping 

 cheeks. The highest point of the glabella is situated about halfway 

 along its length. The anterior part in front of the basal lobes is 

 bent downward and more rounded and convex than the posterior 

 part, which is slightly flattened. The length of the glabella is equal 

 to the width, and the quadrate appearance is due to the width at 

 the base being nearly as great as that across the middle, the axal 

 furrows curving inward only very slightly behind the middle. 

 The height is equal to about half the length. The frontal lobe is 

 very short, being only about one-fourth the length of the glabella, 

 owing to the forward position of the first lateral furrows. These 

 furrows, which arise almost at right angles to the axal furrows and 

 opposite the ' terminal pit/ are curved gently backward, and their 

 free ends are separated by an interval at least twice as great as 

 their length. They are only faintly marked and narrow. 



1 Schmidt, ' Eev. d. ostbalt. Silur. Trilob.' pt. i. p. 171, pi. x. figs. 1-4, & 

 pi. xvi. figs. 22-27, Mem. Acad. Imp. Sci. St. Petersb. ser. 7, vol. xxx. (1881) 

 no. 1. 



