36 



SHELVE DISTRICT 



Pseudolingula spatula sp. nov. 

 (PI. 4, figs. 6-14 ; PL 5, fig. 1 ; Text-fig. 4) 



Diagnosis. Subequally biconvex, subquadrate Pseudolingula with a brachial 

 valve 67% as wide and 5% as deep as long, ornamented by strong fila laterally, 

 becoming finer medially, and bearing an adnate ventral muscle platform and a strong 

 dorsal median ridge arising up to one-fifth anteriorly of the beak and extending 

 forwards 58% of the length of the valve. 



Description. Subequally biconvex, subquadrate Pseudolingula with parallel 

 lateral and obtusely rounded anterior margins and obtuse beaks with the more 

 rounded dorsal one subtending an angle of about ioo° ; brachial valve 67% as wide 

 and about 5% as deep as long ; both valves subcarinate postero-medially but 

 flattening laterally and anteriorly ; external surface ornamented by growth lines 

 and fila which are strongly developed with a wavelength of about 0-2 mm laterally 

 but tending to become finer medially. 



Ventral interior with median depression to accommodate pedicle but nature of the 

 propareas unknown ; umbonal muscles divided and represented by elongately oval 

 scars on either side of the median depression, other muscle bases not identifiable 

 individually but supported on a broad medially pointed platform, heavily rutted by 

 growth lines and extending forward of the beak for about two-fifths of the length of 

 the pedicle valves. 



Dorsal interior with median depression at the beak but posterior margin un- 

 differentiated ; median septum arising up to one-fifth of the valve length anterior of 

 the beak and becoming strong and relatively high at the anterior end which is, on 

 average, 58% of the valve length forward of the dorsal beak (range 54% to 61% for 



pedicle groove 



umbonal scar 



muscle scar 



median septum 



A B 



Fig. 4. Diagrammatic views of (A) the ventral and (B) the dorsal interiors of 



Pseudolingula. 



