ORDOVICIAN BRACHIOPODA 39 



Horizon and localities. Mytton Flags : BB 35472 from adit entrance in 

 Maddox's Coppice, 1200 yds ENE of St Luke's Church, Snailbeach (Grid Ref . 

 SJ 382030) ; BB 35468 from sides of adit running into hillside at New Perkin's 

 Level (Grid Ref. SJ 376022) ; BB 35469 from runnel just east of footpath to Blake- 

 moorflat at head of Mytton Batch (Grid Ref. SJ 373006), BB 35470 from exposures 

 in sunken cart track, west side of Snailbeach Coppice (Grid Ref. SJ 375026) ; 

 BB 35471 from near top of steep tributary to Crowsnest Dingle 350 yds WNW of 

 Blakemoorflat (Grid Ref. SJ 373008) ; BB 35467 from dump below Old Perkin's 

 Level, Snailbeach (Grid Ref. SJ 376022) ; BB 35473 from dump outside adit in 

 Perkin's Beach just below col separating Perkin's Beach and Mytton Batch (Grid 

 Ref. SJ 366002). 



Discussion. The type species of Monobolina has been well known for over a 

 century as an easily distinguishable inarticulate with a restricted stratigraphic and 

 geographic range. Salter (1859 : 56) based the species on specimens collected from 

 Mytton Flags exposed west of the Stiperstones. Most of the specimens used in 

 preparing this emended description are topotypic with his, and the remainder come 

 from localities in the vicinity. They therefore afford an opportunity to present some 

 reliable data on the morphologic variability of the species which illustrate the homo- 

 geneity as well as the distinctiveness of the taxon. 



Table 20 



Statistics of length (1) and maximum width (w) of 19 brachial valves of Monobolina plumbea 



(Salter) 



1 mm (var 1) 8-82 (20-311) 



w mm (var w) 11-29 (3°'383) 



r 0-979 



a (var a) 1-2232 (0-00362) 



Table 21 



Statistics of length (1) and thickness (th) of 18 brachial valves of Monobolina plumbea (Salter) 



1 mm (var 1) i°'37 (16-106) 



th mm (var th) 1-39 (0-487) 



r 0-927 



log e l (var log e l) 2-2688 (0-1398) 



logeth (var log e th) 0-216 (0-2251) 



r e 0-934 



a (var a) 1-269 (0-0128) 



Table 22 



Statistics of length (1) and length of the lateral-transmedial muscle scars (sc) of 19 brachial valves 



of Monobolina plumbea (Salter) 



1 mm (var 1) H'57 (9*2) 

 sc mm (var sc) 5-16 (2-163) 



r 0-903 

 a (var a) 0-4849 (0-00255) 



