ORDOVICIAN BRACHIOPODA 151 



an evenly and gently convex transverse profile ; ornamented by fine closely spaced 

 lamellae and rounded ribs composed of 9 primary costae and intercalated costae 

 within 1 mm of the umbo and costellae appearing in later stages of growth by branch- 

 ing and intercalation with counts of 3 ribs per mm, 5 mm antero-medially of the 

 umbones of 2 pedicle valves ; ventral interarea short, almost orthocline, with a narrow 

 open delthyrium ; brachial interarea not exposed. 



Ventral interior with simple node-like teeth supported by long parallel dental 

 plates extending forward for an average of one-fifth the length of 3 pedicle valves to 

 define a narrow delthyrial chamber ; muscle and mantle canal impressions unknown. 



Dorsal interarea with small septalium supported by a median ridge extending 

 forward for about one-fifth the length of the brachial valve ; muscle and mantle 

 canal impressions unknown. 



Figured material. , ,, . ,,, , . 



length width (mm) 



External and internal moulds of orthocone containing im- 

 pressions of 4 Euorthisina (BB 35377a, b) 

 Incomplete external and internal moulds of pedicle valve 

 (BB 35378a, b) 4-5 



External and internal moulds of pedicle valve (BB 35379a, b) 3-2 - 



Incomplete external and internal moulds of brachial valve 

 (BB 35380a, b) 4-5 7-5 



Horizon and localities. Mytton Flags : BB 35377, 35378 from the adit in 

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

 SJ 382030) ; BB 35379, 35380 from outcrops by the side of the cart-track 140 yds 

 north of Wood House, Gravels (Grid Ref. SJ 338003). 



Discussion. The small number of moulds of Euorthisina collected from the 

 Mytton Flags include a group of 4 poorly preserved conjoined shells all orientated 

 in the same way within an orthocone and probably buried in position of growth. 

 The sample compares most closely with E. moesta minor Havlicek from the Llan- 

 virnian Sarka Shales of Czechoslovakia, especially in the relative dimensions and 

 size of the dental plates (cf. PI. 28, figs. 4, 6, 7). Havlicek (1950 : 87) reports that 

 in his subspecies there are 8 to 12 ribs in the umbonal region compared with 14 to 16 

 in the Shelve specimens. But this difference may be an expression of variability 

 in the older stock and its systematic importance can only be satisfactorily assessed 

 when more material is available. 



Family PARASTROPHINIDAE Ulrich & Cooper 1938 



PARASTROPHINELLA Schuchert & Cooper 1931 



Parastrophinella musculosa sp. nov. 



(PL 28, figs. 9-13, 17) 



Diagnosis. Small, subtriangular, strongly biconvex Parastrophinella with vesti- 

 gial fold and sulcus and commonly 11 to 13 costae, with a wavelength of 0-6 mm, 

 ornamenting the anterior part of the shell. 



