64 SHELVE DISTRICT 



gently convex, transversely semi-oval brachial valve just over two-thirds as long as 

 broad and lacking a sulcus. The ventral interarea was strongly apsacline and, at 

 one-eighth the length of the pedicle valve, longer than the anacline dorsal interarea ; 

 both delthyrium and notothyrium were open. The ornamentation consisted of 

 microscopic concentric lamellae superimposed on fine costellae numbering 4 and 5 

 per mm at 5 mm antero-medially of the umbones, with costellae, mainly branching 

 internally, arising early so that about 60 occur at the 5 mm growth stage and 75 at 

 the margin of the brachial valve 8-5 mm long. 



A pair of dental plates extending forward for about one-seventh the length of the 

 pedicle valve defined the umbonal cavity which was occupied by a subtriangular 

 muscle field with a relatively broad median adductor track ; only proximal traces of 

 the vascula media, separated by a short median ridge, are preserved. 



The cardinalia consisted of a narrow ridge-like cardinal process and sharp, 

 divergent, unsupported brachiophores extending forward for one-fifth of the length 

 of the brachial valve ; only faint impressions of a pair of suboval adductor scars are 

 preserved on either side of a low median ridge becoming indistinct anteriorly. 



The fineness of the ribbing as well as the internal features indicate that the 

 Whittery Dolerorthis resembles D. tenuicostata (Williams 1955 : 406) from the Derfel 

 Limestone of N. Wales. There are, however, some differences especially in the 

 lesser convexity of the brachial valve of the Whittery species which also lacks a 

 sulcus. These differences may ultimately prove to be taxonomically important. 



Subfamily GLYPTORTHINAE Schuchert & Cooper 1931 



GLYPTORTHIS Foerste 1914 



Glyptorthis viriosa sp. nov. 



(PI. 10, figs. 6, 8, 9, 11, 12, 14, 15 ; PI. 11, figs. 1, 2, 4) 



Diagnosis. Small Glyptorthis with a shallowly sulcate brachial valve about four- 

 fifths as long as wide and a catacline pedicle valve 37% as deep as long, ornamented 

 by fascicostellae numbering 2 or 3 per mm, 5 mm antero-medially of the dorsal 

 umbo, and concentric lamellae with a modal density of 3 per mm between 5 and 6 mm 

 antero-medially of the dorsal umbo. 



Description. Subcircular, ventribiconvex Glyptorthis with roundly obtuse 

 cardinal angles ; pedicle valve sharply carinate and 37% as deep as long, brachial 

 valve 80% as long as wide and 23% as deep as long with a persistent rounded median 

 sulcus, up to half as wide as the valve length, flanked by evenly convex lateral areas ; 

 ventral interarea catacline to slightly apsacline, about one-third as long as the valve, 

 with narrow delthyrium with subparallel boundaries ; dorsal interarea anacline 

 with an open notothyrium ; radial ornamentation fascicostellate with ribs branching 

 almost exclusively internally in the first four sectors and numbering 2 and 3 per mm, 

 5 mm antero-medially of the umbones of 3 brachial valves in each case ; concentric 

 ornamentation consisting of strongly developed lamellae with counts of 3 and 4 in 

 3 and 1 valves between 5 and 6 mm antero-medially of the umbones. 



