Maryland Geological Sukvey 305 



Ambocoelia praeumiona Clarke, 1903, N. Y. State Mus., Bull. G5, p. 188. 

 Ambocoelia praeum'bona Grabau and Shimer, 1909, N. Am. Index Fossils, 

 vol. i, p. 342, figs. 438a, b. 



Description. — Shell obliquely semiglobose or ovoid, the liinge-line less 

 than the width of the sliell; cardinal extremities rounded; surface with- 

 out plications. Ventral valve extremely convex or ventricose, with a 

 shallow impressed line which has nearly the character of a narrow shallow 

 sinus; beak large and incurved; cardinal area of moderate dimensions, 

 proportionally high, with the lateral margins not defined; fissure par- 

 tially covered by an arching pseudo-deltidium. Dorsal valve moderately 

 and evenly convex, usually without mesial fold or sinus, the cardinal ex- 

 tremities rounded; sometimes there is a narrow impressed line down the 

 center of the valve. Surface essentially smooth, or marked only by con- 

 centric striae, which are sometimes crowded into imbricating folds; the 

 impression of the ventral valve has a large often pustulose muscular 

 area, margined by a thickening of the shell. In many specimens the 

 length and width are nearly equal, the dimensions being about three- 

 fourths of an inch. 



A large internal impression of a ventral valve together with a few other 

 imperfect specimens in the Maryland Collection are provisionally re- 

 ferred to this species. In the umbonal region is a well marked sinus 

 which becomes broader and shallower toward the front of the shell ; 

 there is also a shallow indistinct furrow between the ridges limiting the 

 sinus and the lateral margin of the shell. Strong concentric wrinkles 

 of growth occur toward the margin and the rather large pustulose mus- 

 cular area is fairly w^ell shown in the umbonal region. In some respects 

 the specimen suggests an unusually large form of Ambocoelia umhonaia 

 (Con.), but it was sent to Prof. Schuchert and he wrote me as follows: 

 "This looks more like A. praeumhona [than .4. umhonata] only the 

 sinus is very much marked. However, this character is somewhat due to 

 its being an internal cast of the shell and may not have been so strong on 

 the outside." On re-examination Prof. Schuchert wrote as follows: 

 ^'At first I was going to label this specimen A. praeumhona but tlie ven- 

 tral sinus is against this identification. However, the specimens are so 



