50 SHELVE DISTRICT 



bounded by short divergent brachiophores and not prolonged anteriorly into a 

 median ridge ; simple narrow sockets were obliquely developed between the hinge- 

 line and the brachiophores. 



The combination of orthacean cardinalia, an unequally parvicostellate radial 

 ornamentation and gently concave profile leave no doubt that the valve must be 

 assigned to Hesperonomia as currently interpreted. Yet the ornamentation is so 

 different from the normal costellate condition of the genus as to bring into question 

 the use of the convexity or concavity of the brachial valve to distinguish between 

 Hesperonomiella and Hesperonomia. Thus although the Mytton specimen is like 

 Hesperonomia australis Ulrich & Cooper (1938 : 116) from the Odenville Limestone 

 of Alabama, it more closely resembles the Arenigian Hesperonomiella carmalensis 

 Bates (1968 : 142) from Anglesey in every respect except that the brachial valve of 

 the Welsh species is very gently convex. Indeed, the Shropshire specimen may have 

 been more closely related to H. carmalensis than to American Hesperonomia because 

 the profile of the brachial valve was not as stable as its use for generic identification 

 implies. 



Family ORTHIDAE Woodward 1852 



Subfamily ORTHINAE Woodward 1852 



ORTHIS Dalman 1827 



Orthis cf. callactis Dalman 1828 



(PL 7, figs. 10, 11, 13-16) 



Diagnosis. Transversely semi-oval, mucronate Orthis with a pedicle valve over 

 one-third as deep as long and a planar brachial valve 57% as long as wide, orna- 

 mented by about 17 costae with a wavelength of about o-8 mm, 5 mm antero- 

 median^ of the dorsal umbo. 



Description. Plano-convex, transversely semi-oval Orthis with sharply acute 

 cardinal angles and a pedicle valve, over one-third as deep as long, evenly convex in 

 longitudinal profile but tending to become flattened postero-laterally ; mean length 

 of 3 brachial valves 57% (range 52% to 60%) of the maximum width, slightly convex 

 for the first 2 mm of growth with a shallow median sulcus but becoming plane and 

 rectimarginate in later stages of growth ; ventral interarea short, about one-twelfth 

 as long as the pedicle valve, strongly curved, apsacline ; dorsal interarea anacline, 

 cardinal extremities acute at about 6o° ; ornamentation consisting of about 17 

 angular costae, about 0-8 mm in wavelength 5 mm anterior of dorsal umbo, bearing 

 fine lamellae and capillae ; postero-lateral sectors with about 5 short rugae disposed 

 at an acute angle with the hinge-line. 



Ventral interior with teeth supported by short dental plates extending anteriorly 

 for about one-seventh the length of the pedicle valve, pedicle callist small, muscle 

 field with a rounded anterior boundary about one-quarter as long as the pedicle 

 valve, adductor scar wide and not enclosed by the submedial diductor lobes. 



