ORDOVICIAN BRACHIOPODA 45 



Dorsal interior with faint median ridge ; posterior adductor scars large, suboval, 

 slightly divergent and commonly with well-defined tracks, extending anteriorly of 

 the posterior margin for 32% of the length of the valve ; anterior adductor scars 

 small, suboval, normally obscure, situated about half-way along the valve and more 

 medially placed than posterior pair ; a pair of short submedial fine ridges located 

 forward of the anterior adductor scars may also represent muscle supports ; pedicle 

 valve unknown. 



Type and figured material. 



length width (mm) 



Holotype External and internal moulds of brachial valve 



(BB 35358a, b) il-o 13-5 



Paratypes Exterior of brachial valve (BB 35359) 9-0 io-o 



External and internal moulds of brachial 

 valve (BB 35360a, b) 1-9 2-3 



Exterior of brachial valve (BB 35361) 3-5 3-5 



External and internal moulds of deformed 

 brachial valve (BB 35362a, b) 7-5 6-5 



External and internal moulds of brachial 

 valve (BB 35404a, b) 4-2 5-0 



Horizons and localities. BB 35358, 35359 from Spy Wood Grit exposures 

 1100 yds NNE of Rorrington (Grid Ref. SJ 303015) ; BB 35360 from Rorrington 

 Beds exposed in Deadman's Dingle 100 ft upstream from its junction with Spy Wood 

 Brook (Grid Ref. SO 280960) ; BB 35361, BB 35362 from Rorrington Beds exposed 

 in tributary to Lower Wood Brook, 340 yds south-east of Desert (Grid Ref. 

 SJ 308017) ; BB 35404a, b from Betton Beds exposed in a stream 330 yds WSW of 

 Little Weston (Grid Ref. SO 290983). 



Discussion. Brachial valves of Schizomania occur rarely in the pre-gracilis 

 rocks of the Shelve area, but are fairly common in the Rorrington Beds and Spy Wood 

 Grit which have provided the two small samples used to derive the statistics given 

 in Tables 27-29. Comparisons of these estimates of dorsal outline and depth and 

 of the anterior extension of the posterior adductor scars show a significant difference 

 (0-05 > p > 0-02) only in the relatively greater depth of the Spy Wood valves. 

 This difference may reflect the growth of a consistently shallower valve during 

 Rorrington time. Two other smaller samples from the Betton and Meadowtown 

 Beds also differ from the Spy Wood specimens only in the depth of the brachial 

 valve which averaged 19% and 20% of the length of 4 and 5 brachial valves respec- 

 tively. However, many Schizocrania from the Betton, Meadowtown and Rorrington 

 Beds show signs of collapse induced by compaction of the shaly sediments in which 

 they occur ; and although there are no obvious signs of deformation in the specimens 

 selected for measurement, the depressed condition of the valves which were mostly 

 thin-shelled may yet prove to have been diagenetically induced. 



Apart from the information provided in Tables 27-29, the two samples are also 

 alike in the rounded outline of the posterior margin and in details of the dichotomizing 

 radial ornamentation. Thus between 4 and 5 mm anterior of the umbo, 7, 8, 9, 10, 



