Maryland Geological Survey 543 



Subgenus POSTLIGATA n. subgen. 



Type. — Glycymeris (Postligata) wordeni. 



Shell lenticular ; umbones inconspicuous, submedial in position ; lunule 

 and escutcheon not defined ; concentric sculpture feebly developed ; liga- 

 ment opisthodetic, lodged in a few grooves oblique to the dorsal margin; 

 bine taxodont, the teeth arranged in an asymmetric but continuous series ; 

 anterior portion of the series feebly concave, the component teeth sub- 

 equal and relatively large and few in number; the posterior portion of 

 the series feebly convex and subparallel to the dorsal margin, the distal 

 teeth larger than those beneath the umbones ; adductor muscle scars sub- 

 equal, two in number ; pallial line simple. 



Postligata shares the characters of Glycymeris and Limopsis. As in 

 both of these genera the shell is compressed and subcircular in outline, 

 dimyarian, and the muscle scars connected by a simple pallial line. It 

 further suggests Limopsis in the asymmetric arrangement of the teeth and 

 the discrepancies between the anterior and posterior dental series. Post- 

 ligata is isolated, however, in the ligament characters. In Limopsis this 

 is multivincular and amphidetic. In Postligata the ligament is alivincu- 

 lar as in Glycymeris, but placed entirely behind the umbones. Known 

 only from the type species from the Monmouth of Maryland. 



Glycymeris (Postligata) wordeni n. sp. 

 Plate XXI, Figs. 7-9 



Description. — Shell small, subcircular in outline, lenticular; umbones 

 small, inconspicuous, submedial, feebly inflated and feebly prosogyrate; 

 lunule and escutcheon not defined ; dorsal margins gently sloping, the 

 anterior slope even lower than the posterior ; anterior lateral margin very 

 broadly rounded, posterior lateral margin squarely truncate ; base-line 

 strongly arcuate ; external surface very finely and regularly striated con- 

 centrically from the umbones to the base with an occasional strongly 

 defined resting stage ; ligament opisthodetic, lodged in three to five short 

 grooves oblique to the dorsal margin ; hinge taxodont, the teeth V-shaped 



Etymology: Post, behind; ligatus. bound. 



