536 Systematic Paleontology 



Arca obesa (Whitfield) Weller 



Cibota obesa Whitfield, 1885, Mon. U. S. Geol. Survey, vol. ix, p. 93, pi. xi, 



figs. 30, 31. 

 Cibota obesa Johnson, 1905, Proc. Acad. Nat. Sci., Phila., p. 9. 

 Arca obesa Weller, 1907, Geol. Survey of New Jersey, Pal., vol. iv, p. 409, 



pi. xxxiv, fig. 9. 



Description. — " Shell small, with full arid very ventricose valves, large 

 tumid beaks situated opposite the anterior third of the length, slightly 

 enrolled, and distant from each other as shown on the internal cast. Form 

 of the outline trapezoidal, the length of the cast nearly twice the height, 

 exclusive of the projection of the beaks; anterior end vertically rounded; 

 posterior obliquely truncate ; extremity obtusely pointed ; basal line full, 

 but constricted just anterior to the middle by the very marked but short 

 and broad byssal opening; area two-thirds the length of the valve and 

 moderately wide. On the casts the muscular imprints are very distinctly 

 marked and of fair size, no muscular ridge ; the outer margin indicating a 

 strong and abrupt thickening of the valves with a crenulated border; 

 radiating lines indicating moderately fine stria? show on nearly all parts 

 of the cast, but strongest on the postero-basal section. 



" The general form of this species is like a dwarfed and extremely ventri- 

 cose specimen of C. uniopsis Conrad, but is so perfectly neat and sym- 

 metrical in its shape as to preclude the idea of a stunted individual. The 

 valves are, however, equally ventricose. while those of that species usually 

 are slightly unequal and sometimes very decidedly so. The form of the 

 byssal opening is also peculiar, being broadly oval and regular instead of 

 a long narrow slit, as is usual." — Whitfield, 1885. 



A single battered cast is the sole representative of the species in Mary- 

 land. 



Cibota Browne, the genus to which Whitfield assigned this species, was 

 isolated by its author because of the presence of a byssus extruded through 

 a gape in the ventral margin of the valves. This is not a character of even 

 subgeneric value, since many of the true Arcce, notably A. noce Lamarck, 

 the type of the genus, are byssiferous. 



Occurrence. — Matawan Formation. Ulmstead Point, Anne Arundel 

 County. 



