74 SHELVE DISTRICT 



long as the valve, the cardinal process a fine median ridge on a low notothyrial plat- 

 form, separating two subtriangular diductor impressions. The short divergent 

 brachiophores were pointed with low convergent bases bounding the notothyrial 

 platform, while the narrow sockets were defined by well-developed fulcral plates. A 

 linear median ridge extended anteriorly of the notothyrial platform for about two- 

 fifths of the valve length, between suboval, poorly impressed adductor scars. 



Two sets of external and internal moulds (BB 35338a, b ; BB 35339) from the 

 Stapeley Volcanic Group collected from outcrops in an adit near Perkin's Beach, 

 west of Shepherd's Rock, represent pedicle valves which appear to be conspecific 

 with the brachial valve described above. The ornamentation, consisting of faint 

 concentric lamellae and rather widely separated sharply rounded hollow costellae, 

 is identical. Unfortunately little can be said about the internal morphology. In 

 the larger specimen, which is 4 mm long, the ventral muscle field is not impressed 

 and only the mould of a divergent dental plate, extending forward for about one- 

 seventh the length of the mould, is seen. 



The profiles of the valves, the development of hollow costellae and, with regard to 

 the brachial valve from the Mytton Flags, the nature of the cardinalia are all 

 indicative of the plectorthid affinities of the specimens. The simplicity of the car- 

 dinal process further suggests that the brachial valve, at least, is closer to Desmorthis 

 than any other genus assigned to the family. However, the convergence of the 

 brachiophores and the lamellose exterior hint that when the species is fully known it 

 will prove to be generically distinct from other plectorthids. 



GELIDORTHIS Havlicek 



Gelidorthis cf. partita (Barrande) 



(PI. 12, figs. 5-7) 



1879 Orthis partita Barrande : plate 63, fig. II. 



1950 Giraldiella partita (Barrande, 1879) Havlicek : 124. 



1968 Gelidorthis partita (Barrande, 1879) Havlicek : 125. 



1971 Gelidorthis partita (Barrande, 1879) Havlicek : 42. 



A small number of impunctate valves never more than 8 mm long, from the 

 Rorrington Beds exposed in Spy Wood Brook 20 yds downstream from where it 

 joins Deadman's Dingle (Grid Ref. SO 289958), most closely compare with Gelidorthis 

 partita (Barrande) from the Lower and Middle Caradoc rocks of Czechoslovakia. 

 The shells were subquadrate in outline with an evenly convex pedicle valve 

 (BB 37159 ; BB 35418a, b) about three-quarters as long as wide and about one- 

 eighth as deep as long, and a very gently convex brachial valve (BB 35419a, b) 

 lacking a definite sulcus. The relatively coarse radial ornamentation consists of 

 angular costellae numbering 3 per mm at 5 mm antero-medially of the ventral umbo. 

 The ribbing is strongly impressed internally even in the ventral umbonal cavity 

 which is defined by short, widely divergent dental plates extending forward for about 

 one-tenth the length of the pedicle valve. As in the pedicle valve, the dorsal muscle 

 field is also obscured by internal ribbing ; but the cardinalia are well developed and 

 consist of a cardinal process incipiently differentiated into a myophore and shaft, 



