ORDOVICIAN BRACHIOPODA 119 



Horizon and locality. Spy Wood Grit exposed 1100 yds north-east of 

 Rorrington (Grid Ref. SJ 303015). 



Discussion. Bicuspina is well represented in the Spy Wood Grit and the 

 Whittery Shales by two closely related forms which are, nonetheless, distinguishable 

 in the relative size of the median fold and sulcus and in ornamentation. As com- 

 parison of growth axes demonstrated (Tables 86-93), the two species are alike in the 

 outline and depth of the valves as well as in the relative growth of dental plates and 

 cardinalia. The dorsal fold of B. subquadrata, however, is relatively narrower than 

 that of B. modesta, and a /-test indicates that the difference was maintained during 

 shell growth at a significant level (o-oi > p > o-ooi). This difference may have 

 influenced the incidence of ribs on the folds because a median costa is more fre- 

 quently absent or, if present, more likely to have bifurcated in later stages of shell 

 growth in B. subquadrata than in B. modesta. Concentric lamellae of the former 

 species are also more delicate and more closely spaced than those of B. modesta. 



The Shelve species may be ancestral to B. spiriferoides (M'Coy) which is widely 

 distributed in late Soudleyan and Longvillian rocks of Shropshire and N. Wales. 

 Lack of suitable material has precluded study of the variability of the radial ornament 

 and especially the development of a dorsal median costa (Williams 1963 : 426). B. 

 spiriferoides, however, is distinct in its relatively greater width, its rectangular to 

 acute cardinal angles, and its relatively long dental plates and ventral muscle field. 



The Bicuspina described by Havlicek (1950 : 88-90) from the late Llandeilian- 

 early Caradocian (Chrustenice and Lodenice substages) rocks of Czechoslovakia also 

 differ in a number of respects but especially in the relative coarseness of ribbing in 

 B. cava (Barrande) and the lack of fold and sulcus in B. multicostellata Havlicek. 



Table 86 



Statistics of length (1) and maximum width (w) of n brachial valves of Bicuspina subquadrata 

 sp. nov. (A) and Bicuspina modesta sp. nov. (B) 



A B 



n 7 34 



1 mm (var 1) 9-10 (10-037) °"55 (8-024) 



w mm (var w) 13-94 (19-843) 8-70 (13-113) 



r 0-986 0-966 



a (vara) 1-4061 (0-01130) 1-2783 (0-03384) 



Table 87 



Statistics of length (1) and maximum depth (th) of n brachial valves of Bicuspina subquadrata 

 sp. nov. (A) and Bicuspina modesta sp. nov. (B) 





A 



B 



n 



1 mm (var 1) 



th mm (var th) 



r 

 a (var a) 



10 



8-13 (4*447) 



3-00 (1-315) 



0-916 



o-5439 (0-00595) 



33 



6-70 (7-165) 

 2-36 (1-033) 

 0-869 

 o-3797 (0-00114) 



