48 SHELVE DISTRICT 



valve is located submarginally. In other features, however, especially the trans- 

 versely oval outline and the absence of fila, the new species is unlike penecontem- 

 poraneous Schizotreta including the type species of the genus S. elliptica (Kutorga) 

 from the Baltic area (Goryanski 1969 : 83), and Scottish and American stocks 

 (Williams 1962 : 94-95 ; Cooper 1956 : 277-282). Only S. microthyris from the 

 Oranda Formation of Virginia compares with S. transversa in outline although fila 

 are finely but persistently developed on the American species and serve to distinguish 

 the two. 



Schizotreta sp. 



(PL 7, %• 4) 



The external mould of a discinid pedicle valve (BB 35580), from the Mytton Flags 

 exposed in the sides of the adit in the New Perkin's Level (Grid Ref. SJ 376022), has 

 been provisionally identified as a Schizotreta rather than an Orbiculoidea mainly on 

 the basis of its elongately oval outline and strong regular fila. The valve was 

 4-5 mm long and about two-thirds as wide and one-fifth as deep as long. The beak 

 was located about one-third the length of the valve anteriorly of the curved posterior 

 margin with the pedicle opening represented by a narrow slit, about 0'5 mm long, 

 breaching the slightly concave posterior surface. 



The valve is immediately distinguishable from 5. transversa in its elongately oval 

 outline and strong fila which, together with the relatively anterior location of the 

 beak and pedicle opening, render the specimen different from most other described 

 species. However, in view of the fact that no Schizotreta has previously been reported 

 from Lower Ordovician rocks (Rowell in Williams et al. 1965 : H285), more informa- 

 tion, especially about the brachial valve, is required before taxonomic recognition 

 is warranted. 



Suborder CRANIIDINA Waagen 1885 



Superfamily CRANIACEA Menke 1828 



Family CRANIIDAE Menke 1828 



PETROCRANIA Raymond 1911 



Petrocrania dubia sp. nov. 



(PI. 7, figs- 5, 6, 8) 



Diagnosis. Petrocrania known only by its subquadrate, conical brachial valve 

 with the greatest depth corresponding more or less to mid-point ; elevated anterior 

 adductor scars larger than posterior. 



Description. Exterior of brachial valve slightly irregular, subquadrate and 

 suboval with flattened posterior margin, 82% as long as wide (mean for 4 valves) 

 and 26% as deep as long (mean for 3 valves) with maximum depth at about mid- 

 point ; surface with sporadic concentric growth lines, otherwise smooth ; shell 

 with thickened slightly everted margin corresponding to limbus about 1 mm wide. 



