SILURIAN OF THE LAKE -DISTRICT. 885 



Beck, belonging to this subgenus, is deserving of some notice. It 

 much resembles P. Stokesii, Milne-Edw., but the glabella is longer 

 as compared with the width, and the axis of the tail is not so flat ; 

 the body- segments also of the axis have large tubercles at their 

 extremities. The chief feature, however, consists of a sort of fringe 

 which is seen round one of the cheeks, the margin of the glabella 

 not being preserved sufficiently to show whether it extended all 

 round. It is formed by a single row of round holes. 



I believe there is also a smaller species of this subgenus in the 

 same bed, but it is not sufficiently perfectly preserved for iden- 

 tification. 



Formation. Graptolitic Mudstones. 



Locality. Skelgill Beck. The smaller species is also found at 

 Spengill and Torver Beck. 



4. Phacops (Odontochile) obttjsicattdattjs, Salter. 



This species, which has a general resemblance to Phacops mucro- 

 natus, var., occurs in the Lake-district proper, almost confined to a 

 single bedding-plane, in the Middle Coldwell Beds, along which it 

 may be traced for many miles. This resemblance is of interest as 

 bearing on evolution ; the glabella is of much coarser structure than 

 that of P. mucronatus, var., whilst the tail is exceedingly obtuse, 

 that of P. mucronatus being very acute and terminated with a long 

 spine. It is very probable that these variations are due to differ- 

 ences of sediment ; for whereas we find P. mucronatus, var., in ex- 

 tremely fine shales, P. obtusicaudatus occurs in coarse grits. In the 

 Upper Coldwell Beds, which are less gritty, I have found a variety 

 of this species in which the side furrows of the tail are slightly bent 

 downwards, and there are some traces of a small spine ; in the finer 

 beds of the higher series in "Wales we get the more strongly spined 

 P. caudatus ; whilst in the very fine Wenlock shale the very long- 

 spined P. longicaudatus is found. 



Formation. Middle Coldwell Beds. {Note. The specimen referred 

 to this species from the Ashgill Shales of the Sedbergh district in 

 Prof. Hughes's paper, Geo!. Mag. vol. iv. no. 8, is the tail of another 

 species.) 



Localities. Many quarries in the neighbourhood of Coldwell ; 

 quarries on both sides of Troutbeck ; north side of Helm Knot, 

 Dent. 



