Vol. 52.] PAUNA OF THE KEISLET LIMESTONE. 423 



Ch. (Ps.) subquadratus measures 17 millim. in length and 32 millim. 

 in width ; the glabella of this specimen measures nearly 15 millim. 

 in length, and the breadth is the same across the middle and 13 

 millim. at the base. The largest specimen that 1 have seen has a 

 head-shield 23'5 millim. in length, but most of them range between 

 16 and 18 millim. 



Note. — A hypostome found unconnected with any head-shield 

 shows some new features, though on the whole it resembles that of 

 Ch. {Ps.) hemicranium figured by Schmidt. It may therefore not 

 improbably belong to this new species Ch. (Ps.) subquadratus. In 

 shape it is broadly ovate, obtusely pointed behind and rounded in 

 front. The central area, which is slightly but regularly convex, 

 has parallel sides and a truncated front end. A rather deep furrow 

 surrounds this area and marks off a tumid border with an average 

 width of about one-sixth of the central area. This border is 

 thickened and more elevated at the posterior end of the hypostome, 

 but at the anterior end it is indented by a deep notch on each 

 side, and produced into a tapering ' ascending process ' with a broad 

 base as in typical species of Cheirurus. The whole surface of the 

 hypostome is granulated, and on the convex central area are also 

 some scattered pits. The length of the hypostome is 8*5 millim., 

 and the breadth 7 millim. across the middle. 



SpH^EEOCOEYPHE GEANULATA, Allgelin ? 



A small globular glabella showing a little nodular lobe at the 

 base and a portion of the fixed cheek corresponds, so far as these 

 fragmentary portions permit determination, with Angelin's species 

 Sph. granulata. 1 The ornamentation of the surface is also similar. 



Sphjeeexoches mieus, Beyr. 



There occur in the Keisley Limestone numerous heads exactly 

 like those of this well-known trilobite. I have seen only one typical 

 pygidium from this bed,, and it is quite probable that some of the 

 heads belong to the species I have called Sph. latirugatus, for the 

 pygidia of this latter species are associated with the heads of the 

 so-called Sph. mirus. I am unable to make two species out of the 

 head-shields, though variations in the distance apart of the basal 

 lobes, etc., do occur, as Salter himself has remarked. 



SPH^EEXOCHUS LATIEUGrATUS, sp. n. (PI. XX. fig. 12.) 



A pygidium has recently been found which, while undoubtedly 

 belonging to the genus Splicer exochus, presents many points of 

 difference from the common British species Sph. mirus. It resembles 

 lands trom's * >S^j7i. lacinittus more closely than any other species 



1 Angelin, ' Pakcont. Scaud.' 1854, p. 76, pi. xxxix. fig. 4 ; Schmidt, ' Rev. 

 (1. osibalt. Silur. Trilob.' pt. i. pi. viii. figs. 17-19, p. 109, Mem. Acad. Imp. tici. 

 St. Petersb. ser. 7, vol. xxx. (1881) no. 1. 



2 Lindatrom, « Forteckn. pa Gtotl. Silur. Crustac' Off. k. sv. Vet.-Akad. 

 Forhandl. 1885, no. 6, p. 47, pi. xiii. fig. 6. 



a J. G. S. No. 207. 2 a 



