364 SYSTEMATIC PALEONTOLOGY 



Genus LITHOPHAGA Bolten. 

 LiTHOPHAGA SUBALVEATA Conrad. 



Plate XCVII, Fig. 1. 



Lithophaga unhalveata Conrad, 1866, Amer. Jour. Conch., voL ii, p. 73, pi. iv, fig. 4. 

 Litliophaga subalveata Whitfield, 1894, Mon. xxiv, U. S. Geol. Survey, p. 40, [pi. v, 

 fig. 9. 



Description. — " Oblong, very thin and fragile, ventricose, posterior side 

 produced, a slight wide furrow marks the umbonal slope, on and behind 

 which are concentric grooves and lines; basal line slightly emarginate 

 or contracted." Conrad, 1866. 



A single broken valve show? a produced posterior side with a slight, 

 wide, flat furrow on the umbonal slope crossed by concentric grooves 

 with posterior end narrow and somewhat bluntly rounded. From a 

 comparison with Conrad's broken and poorly patched type in the 

 Academy of Natural Sciences, the two shells seem to be the same. 



Occurrence. — Calvert Formation. 3 miles west of Centerville. 



Collections. — Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences. 



Lithophaga ionensis n. sp. 

 Plate XCVII, Figs. 2, 3. 



Description. — Shell very thin and fragile, anterior end rounded, 

 posterior region broadened, posterior end rounded; external surface 

 either smooth or concentrically wrinkled across the umbonal slope; 

 ventral margin slightly convex; dorsal margin straight to the posterior 

 end of the hinge line, then rounded and declining; within, a slight 

 submarginal dorsal thickening or ridge just beneath, and extending 

 the length of, the hinge and minutely grooved for the ligament ; beak 

 not prominent; no sulci. 



Some specimens are less inflated and posteriorly broadened and more 

 cylindrical with nearly straight ventral margin, and dorsal margin at 

 posterior end of hinge line more angular than the type. These dif- 

 ferences, however, do not seem to be of enough value to warrant varietal 

 distinction. Specimens are found in the shells of Melina, Ostrea and 

 Pecten, at times riddling these shells by their boring. 



