8 4 



SHELVE DISTRICT 



cardinal process 



socket 

 brachiophore 

 brachiophore base 



median septum 





A 



B 



c 



0-0-9 



5 



1 





1-0-1-4 





2 





1-5-2-0 





1 



4 



Fig. 8. Diagrammatic views of the dorsal interiors of Protoskenidioides showing three 

 stages (A, B, C) in the development of the cardinalia during ontogeny ; with a correla- 

 tion table below indicating the relationship between the length of the brachial valve 

 and the illustrated growth stages of the cardinalia. 



The smallest moulds show that when brachial valves were less than i mm long the 

 brachiophores were well developed and widely divergent (Text-fig. 8). Postero- 

 laterally they were flanked by simple slot-like sockets indented on the valve floor. 

 Antero-medially they were continuous with a pair of transversely oval areas defined 

 by slightly elevated anterior boundaries that converged towards the notothyrial 

 apex of the valve. It is assumed that these wide, discrete areas represented the 

 seats for the attachment of the dorsal ends of the diductor muscles rather than 

 supports for the brachiophores although it is convenient to refer to them as brachio- 

 phore bases. At this growth stage there was no notothyrial platform and the valve 

 interior was dominated by the median elevation representing the external sulcus 

 bearing a small, thin, median septum. 



With further growth the antero-lateral parts of the sockets were raised above the 

 valve floor by the development of concave fulcral plates and the brachiophore bases 

 were extended anteriorly as a pair of lobate platforms which were also free of the 

 valve floor. The bases also encroached onto the median elevation which bore a 

 small ridge in some valves, finally to converge and form a simple cardinal process 

 clearly made up of the adjacent borders of the brachiophore bases and contained 

 within a low, anteriorly bilobed septalium. Concomitantly the median septum 



