126 SHELVE DISTRICT 



(Nikiforova & Andreeva 1961 : 200), is additionally Onychoplecia-like in outline 

 and even more finely ribbed ; while the species described by Opik (1930 : 200) as 

 'Cliftonia' dorsata Hisinger may prove to be a Bicuspina (Roomusoks 1970 : 121) 

 rather than an Oxoplecia. 



The Shelve specimens have been assigned to Oxoplecia nantensis MacGregor based 

 upon material from the Upper Llandeilo of the Berwyn Hills despite the fact that 

 measurements (MacGregor 1961 : 196) indicate the type specimens at least to be very 

 much wider. The significance of this difference cannot be ascertained until more is 

 known about the variability of shell outline. MacGregor's description of 0. nantensis 

 also suggests that the Shelve specimens have a greater number of ribs on the fold. 

 However a photograph of the holotype {Ibid. pi. 20, fig. 18) shows 6 ribs on the 

 fold in contradiction to an error in description which speaks of '3-5 ribs on the dorsal 

 fold' {Ibid. : 197) ; it is possible that modal rib densities for the Shelve and Berwyn 

 Oxoplecia do not greatly differ. 



Order STROPHOMENIDA Opik 1934 



Suborder STROPHOMENIDINA Opik 1934 



Superfamily PLECTAMBONITACEA Jones 1928 



Family LEPTESTIIDAE Opik 1933 



Subfamily LEPTESTIINAE Opik 1933 



PALAEOSTROPHOMENA Holtedahl 



Palaeostrophomena sp. 



(PI. 2i, figs. 7, io, 11, 13) 



Diagnosis. Semicircular Palaeostrophomena just over half as long as wide, 

 ornamented by unequally developed parvicostellae numbering 8 to 10 per mm, 

 5 mm antero-medially of the umbo, very fine fila and about 8 impersistently con- 

 centric rugae ; ventral muscle scar bilobed divided by divergent vascula media. 



Description. Semicircular Palaeostrophomena, with a pedicle valve 53% as 

 long as wide, acute cardinal angles and a very gently concavo-convex profile flatten- 

 ing or slightly resupinate peripherally ; shell surface ornamented by very fine 

 crowded fila, about 8 concentric rugae with a wavelength of about 0-2 mm strongly 

 developed at acute angles to the hinge-line but becoming subdued anteriorly, and 

 by parvicostellae numbering 8 to 10 per mm, 5 mm antero-medially of the umbo, 

 and divided into narrow sectors by accentuated ribs, 10 or 11 of which arise early in 

 the umbonal area ; ventral and shorter dorsal interareas apsacline and anacline 

 respectively with a small pseudodeltidium and a supra-apical foramen, and a dorsal 

 notothyrium filled by a median ridge representing the posterior surface of a plectam- 

 bonitacean cardinal process. 



Ventral interior with relatively small simple teeth, vestigial dental plates and a 

 bilobed ventral muscle scar extending forward for about one-fifth the length of the 

 valve and about three-fifths as long as wide ; diductor scars extending beyond the 



