122 SHELVE DISTRICT 



Caeroplecia plicata sp. nov. 

 (PI. 20, figs. 14-16 ; PI. 21, figs. i-6, 8 ; Text-fig. 9) 



Diagnosis. Subcircular Caeroplecia with a brachial valve about 80% as long as 

 wide, and low rounded costellae with a wavelength of 0-6 mm, 5 mm antero-medially 

 of the dorsal umbo, first appearing on shells more than 3 mm long and commonly 

 numbering 6 on the fold and 5 to 9 on the lateral slopes. 



Description. Dorsibiconvex, subcircular Caeroplecia with obtusely rounded 

 cardinal angles ; pedicle valve 18% as deep as long with flat-bottomed shallow median 

 sulcus and flattened lateral slopes ; brachial valve 80% as long as wide and 30% as 

 deep as long with flat median fold, 43% as wide as the length of the valve, originating 

 2-5 mm anterior of umbo (average for 4 brachial valves) ; ventral interarea short 

 apsacline, pseudodeltidium narrowly folded medially, dorsal interarea linear ; 

 concentric ornamentation of 8 to 10 fila per mm between 5 and 6 mm anterior of 

 dorsal umbo, radial ornamentation not developed on immature shells up to an average 

 of 3-2 mm long (variance 2-233 f° r I0 brachial valves), thereafter rounded costae and 

 implanted costellae occur, having a mean wavelength of 0-63 mm, 5 mm anterior 

 of the umbones of 4 brachial valves with 5, 6, 7 and 8 ribs on the folds of 2, 3, 1 and 

 2 brachial valves between 9 and 12 mm long and 5 to 9 ribs on the lateral slopes. 



Pedicle tube weakly developed, less than 7% as long as the valve ; teeth supported 

 by acutely divergent dental plates extending anteriorly for a mean of 11 -8% of the 

 length of 6 pedicle valves (variance 5-86) ; muscle field faintly impressed but broadly 

 bilobed, extending anteriorly for about one-third the length of the valve, with splayed 

 diductor scars not enclosing a narrow parallel-sided adductor track divided by a fine 

 median ridge. 



Cardinalia consisting of slightly recurved forked cardinal process, short brachio- 

 phores and obliquely placed sockets arranged along the hinge-line for an average of 

 24% of the length of 5 brachial valves ; adductor scars quadripartite, lightly im- 

 pressed about a fine median ridge. 



- foramen 



cardinal process 



pedicle tube 



socket ridge 

 dental plate 



\ run /, 



& mi 



l 1 \ ' 



gonadal sac 

 - muscle scar 

 -mantle canal 



A B 



Fig. 9. Diagrammatic views of (A) the ventral and (B) the dorsal interiors of 



Caeroplecia. 



