136 SHELVE DISTRICT 



to the hinge-line, in the posterolateral areas. The dorsal interiors of mature valves 

 are further distinguished by the almost invariable presence of a median septum 

 flanked by 3 pairs of septa in various stages of coalescence. The occurrence of a 

 median septum in these stratigraphically older Sowerbyella is especially significant 

 although the timing of its appearance in relation to the submedial septa was different. 

 In the Meadowtown sample, the median septum is at least as strongly developed as 

 the submedial pair even in the smallest valve (1-5 mm long), which suggests that the 

 septa were secreted more or less simultaneously presumably after the transformation 

 of the trocholophous lophophore into a schizolophe. In the Spy Wood sample, on 

 the other hand, the median septum is shorter than the submedial pair, and in 3 moulds 

 less than 2 mm long it was either absent or weak indicating that it developed later 

 than the submedial septa. 



At this juncture it can be stated that the Meadowtown specimens are indistinguish- 

 able from a sample of S. antiqua Jones from Ffairfach flags of the Llandeilo area, 

 not only in the early development of the median septum but in every other mor- 

 phological feature except for the greater length of the ventral muscle scar in 5. 

 antiqua s.s. (p < o-ooi). Although this difference is statistically significant, it 

 seems reasonable to compare the Meadowtown material with the Welsh species. 



The Spy Wood specimens differ significantly from both samples of S. antiqua not 

 only in the late development of the median septum but also in the relatively faster 

 anterior spread of the septa (0-02 > p > o-oi), and in the slower lateral growth of 

 the brachial valve relative to its increase in length (o-oi > p > o-ooi). These 

 differences are sufficient to warrant the recognition of a new species, S. multiseptata, 

 based on the Spy Wood sample. 



The second distinctive group of Sowerbyella includes specimens recovered from the 

 younger Aldress Shales and the Whittery and Hagley Shales. Only the Aldress 

 sample is big enough to indicate the range of septa development. It does, however, 

 demonstrate a further significant reduction in the development of a median septum 

 compared with even its relatively late appearance in S. multiseptata. Only a pair of 

 strong submedial septa occurred in 4 moulds less than 2-5 mm long ; and a short septum 

 had developed in only 3 out of 24 larger valves. No immature specimens of Sower- 

 byella were recovered from the Whittery and Hagley Shales, but traces of a septum 

 were not seen in any of the 4 brachial valves examined which were between 3 and 

 6-6 mm long. 



Comparisons of the Aldress and Whittery samples with described Caradocian 

 Sowerbyella indicate that they are closely comparable with S. sericea (J. de C. 

 Sowerby). The Aldress specimens in particular could be well tested statistically in 

 relation to recently described species (Williams 1963 : 430-447). It was found that 

 they differed from S. sericea permixta Williams only in the relatively wider spread 

 of the ventral muscle scar (p < o-ooi). The Whittery and Hagley specimens, 

 however, are more like 5. sericea s.s. in the coarser parvicostellate ribbing, the wave- 

 length, number and more regular occurrence of the postero-lateral rugae, and the 

 greater relative depth of the pedicle valve. Provisionally, therefore, the strati- 

 graphically youngest Sowerbyella are referred to S. sericea s.s. and the Aldress forms 

 to the subspecies permixta. 



