30 
HIXD : THE MILLSTONE GRIT FAUNA 
LIMATULINA OCCIDENTALIS SHUMARD, sp. 1885. 
[Figs. 5, 6, 7, Plate III. ; Fig. 8, Plate IV.]. 
Pecten occidentalis Sliumard, 1855, Missouri rep., page 207, PI. c. 
Fig. 18. 
,, cleavelandicus Swallow, 1858, Trans. St. Louis Acad. Sci., 
Vol. I., page 184. 
,, cleavelandicus Meek & Hayden, 1864, Pal. Up. Mo., page 50, 
PI. II., Fig. 10. 
occidentalis Meek & Worthen, 1866, Geol. Kep., Illinois, Vol. 
II., page 331., PI. XXVII., Figs. 4 and 5. 
,, missouriensis (?) Geinitz, 1866, Carb. and Dyas in Neb., page 
35, Tab. II., Fig. 18. 
Aviculopecten occidentalis Meek and Hayden, U.S. Geol. Soc, 
Nebraska, page 191, PI. IX., Fig. 10. 
Specific Characters. — Shell unequivalve, the right valve almost 
flat, the left more gibbose, excluding the ears, subovate acute, not 
oblique. The anterior, inferior and posterior margins regularly rounded 
and moderately convex. Ears well marked and large, not extending 
quite as far forwards and backwards as the greatest transverse diam- 
eter of the valve. The anterior ear of the right valve is depressed 
and sharply marked off from the rest of the shell by an oblique groove, 
the posterior triangular, flattened and its posterior border falciform. 
The anterior ear of the left valve is triangular, well defined, and much 
depressed so that the shell from the umbo to the margin forms a marked 
angular ridge. The posterior ear more gradually compressed, and its 
posterior margin falciform. The umbo of the left valve is acute, 
erect, central. 
Interior. — The hinge-plate is flattened and striated transversely. 
Exterior. — The surface of the left valve is ornamented by nu- 
merous somewhat irregular radiating lines, some of which start from 
the umbo and others are intercalated between them as they pass 
across the surface so that it happens that in a group of four the outer 
lines are the strongest, the middle line a little less strong, and the 
intermediate lines still weaker. On the right valve the ribs are coarser 
and less numerous. 
