Vol. 52.] FAUX A OF THE KEISLEY LIMESTONE. 417 



border. The length of this epistome is 9-5 millim., and its width 

 from side to side is 33 millim. 



Cheirurid^:. 



Cheirijrus bimtjcronatus, Murch., var. a, Salter. 



It seems that this species, though occurring in the Keisley 

 Limestone, is not so plentiful as Ch. heisleyensis. There is only a 

 single pygidium belonging to it in the Woodwardian Museum, and 

 no undoubted head-shields. Our specimen belongs to the variety a 

 of Salter, 1 in which the terminal mucro between the third pair of 

 spines is absent. 



Chetrurtjs cancrurtjs, Salter. 



Two pygidia of this species are in the Woodwardian Museum, 

 but call for no special notice. Fragments of others are at Carlisle. 



Note. — A portion of a glabella agrees fairly well with that assigned 

 provisionally by Salter to this species.^ The elongated shape of the 

 glabella, the great length of the frontal lobe, and the finely granu- 

 lated surface are the most striking points of similarity, but the side- 

 furrows in our Keisley specimen are much longer than those 

 described and figured by Salter, for each furrow of the front pair 

 extends across the glabella at least a third of its breadth. The 

 figure given by Tornquist 3 of Ch. insiynis (Beyr.) much resembles 

 our specimen. The glabella is much more inflated and convex than 

 that of Ch. bimucronatus or that of Ch. heisleyensis. Ch. conformis 

 of Angelin 4 may also be compared with it. 



Cheirtjrtjs k:eisleyensis, sp. n. (PI. XX. figs. 7, 8 & 9.) 



This new species is founded on two incomplete pygidia, but some 

 associated head-shields with less peculiar characters most probably 

 belong to it. It seems that some of the head-shields have previously 

 been assigned to Ch. bimucronatus, which they much resemble. But 

 the features presented by the pygidium are very marked, and distinct 

 from any British species previously described. 



The pygidium is about 12 millim. long, and is furnished with 

 three distinct pairs of pleurae with free ends. The fourth pair of 

 pleurae and axal segments are represented by a single median, sub- 

 terminal, triangular piece, embraced laterally and posteriorly by the 

 third pair of pleurae. 



The first pair of pleurae is the largest of the three pairs, both in 

 breadth and length. Each pleura of this pair is composed of an 

 inner attached and an outer free portion. This outer free portion 

 consists of a long spine. The inner portion is at right angles to 



1 Salter, PaL Soc. 'Mon. Brit. Trilobites/ p. 63, pi. vi. figs. 15, 16, & 17. 



2 Ibid. p. 73, pi. v. fig. 16. 



3 Tornquist, ' Undersokn. ofv. Siljans. Trilobitf.' Sver. geol. Undersokn. 

 1884, p. 12, pi. i. fig. 9. 



4 Angelin, 'Palseont. Scand.' 1854, p. 32, pi. xxi. fig. 3; Lindstrom. 

 • Forteckn. pa Gotl. Silur. Crustae.' Ofv. k. sv. Vet.-Akad. Forhandl. 1885, 

 no. 6, p. 45, pi. xiii. figs. 13 & 14. 



