i2 4 shelve district 



Table 95 



Statistics of length (1) and maximum depth (th) of 7 brachial valves of Caeroplecia plicata gen. et 



sp. nov. 



] mm (var 1) 9-81 (10-558) 



th mm (var th) 2-89 (2-381) 



r 0-865 



a (var a) °"474° (0-01136) 



Table 96 



Statistics of length (1) and maximum width of the folds (f) in 6 brachial valves of Caeroplecia 



plicata gen. et sp. nov. 



1 mm (var 1) 9-48 (io-686) 



I mm (var f) 4-12 (2-322) 



r 0-952 



a (var a) 0-4661 (0-00509) 



Table 97 



Statistics of length (1) and maximum depth (th) of 10 pedicle valves of Caeroplecia plicata gen. et 



sp. nov. 



1 mm (var 1) 11-45 (5-679) 



tB mm (var th) 2-02 (0-409) 



r 0-636 



a (var a) 0-2683 (0-00535) 



OXOPLECIA Wilson 1913 

 191 3 Oxoplecia Wilson : 81. 



The distinction between Oxoplecia and Bicuspina is finely drawn. Ignoring the 

 variability normally inherent to the development of a fold and sulcus, and radial 

 and concentric ornamentation, the difference ultimately rests on the spiriferoid 

 outline and pedicle tube of Bicuspina (Wright in Williams et al. 1965 : H358). 

 These features are likely to have arisen by a lateral acceleration in the growth of the 

 cardinal margins and secondary shell accretion around the internal opening of the 

 pedicle foramen respectively, and were not always equally developed in stratigraphi- 

 cally earlier stocks. Thus Llandeilian Bicuspina of Czechoslovakia (Havlicek 1950 : 

 18-19) are strongly orthoid in outline, and contemporaraneous Oxoplecia from S.W. 

 Wales have incipient pedicle tubes (R. Addison, pers. comm.). In effect it is likely 

 that members of the earlier-occurring and longer-ranging cosmopolitan Oxoplecia 

 were ancestral to Bicuspina, which appears to be restricted to the Llandeilian and 

 Caradocian rocks of Europe. It is even possible that the species described below is 

 more closely related to Bicuspina than contemporaneous American Oxoplecia which 

 are typically elliptical in outline, strongly uniplicate, lamellose and coarsely costellate. 



