54 SHELVE DISTRICT 



more than 5 mm long, with a mean wavelength (and variance) of o-8i mm (0-005), 

 5 mm anterior of the umbones of 7 brachial valves, crossed by fine lamellae and 

 perforated by a row of exopuncta on either side of each costal crest in both valves ; 

 ventral interarea curved, apsacline, about one-sixth as long as the pedicle valve ; 

 dorsal interarea anacline, shorter, cardinal extremities invariably obtuse in shells 

 longer than 3 mm. 



Ventral interior with blunt teeth rarely crenulated, supported by narrowly diver- 

 gent dental plates extending anteriorly for 23% of the length of the pedicle valve, 

 pedicle callist small, ventral muscle field cordate in adult valves 84% as wide as 

 long with the submedial diductor lobes extending anteriorly for 36% of the length of 

 the pedicle valve but not enclosing the shorter, medially divided, adductor scar ; 

 ventral mantle canal system saccate with proximal parts of vascula media adjacent. 



Dorsal interior with ridge-like cardinal process, oblique sockets and short, rod-like 

 brachiophores usually heavily encased in secondary shell, brachiophore bases 

 extending anteriorly for 19% of the length of the brachial valve and 48% as long as 

 the lateral extension of the simple sockets ; dorsal adductor field quadripartite 

 extending forward of the umbo for 51% of the length of the valve ; dorsal mantle 

 canal system not completely known but probably saccate. 



Type material. 



Holotype Exterior of brachial valve (BB 35371) 

 Paratypes Exterior of brachial valve (BB 35372) 



Internal mould of brachial valve (BB 35373) 



Internal and external moulds of brachial valve 



(BB 35374a, b) 



Internal mould of pedicle valve (BB 35375) 



Pedicle valve and internal mould (BB 35376a, b) 



Horizon and localities. Whittery Shales : BB 35374 from exposures in the 

 lower part of the west bank of the River Camlad 60 yds north-east of Marrington 

 Farm (Grid Ref. SO 272970) ; BB 35375 from exposures in Whittery Quarry at the 

 south end of Whittery Wood, near Chirbury (Grid Ref. SO 275981) ; all other 

 specimens from outcrops in a stream at the north end of Spring Coppice 865 yds 

 south-east of Hockleton Bridge, near Chirbury (Grid Ref. SO 279997). 



Discussion. Like Lenorthis proava the Whittery Orthambonites is characterized 

 by a shell that did not deviate significantly during growth from the modal orthid 

 shape and proportions, even to the development of obtuse cardinal extremities in 

 shells only 2-5 mm long (Tables 30-38). Moreover, although the new species 

 differs from most congeneric stocks, including the penecontemporaneous American 

 0. bellus (Cooper 1956 : 296) and 0. friendsvillensis (Cooper 1956 : 303), in its sig- 

 nificantly fewer ribs, in that character and all others that can be tested statistically 

 it is indistinguishable from the Scottish Caradocian 0. playfairi (Reed emended 

 Williams 1962 : 97). The Whittery shells, however, are unique among all known 

 Orthambonites in bearing a row of exopuncta on either side of each costa of both 

 valves. 



length 

 8-5 



width (mm 



10-2 



7.4 

 7-8 



9-0 

 9-6 



4'5 

 8-o 



8-5 



8-5 

 9-0 



