Beede.] Carboniferous Invertebrates. 101 



and behind the teeth. In other specimens the posterior of the 

 shell is much thickened, the muscular impression not extending 

 beneath the beak or hinge line, that portion of the cavity being 

 tilled with shell, which has encroached upon the muscular area 

 until it is small and nearly circular. This heavy deposit of 

 shell also forms strong supports for the teeth, dividing the pos- 

 terior cavity into a right and a left cavity. 



AMBOCCELIA. 



Hall, 13th Rep. N. Y. St. Cab. Nat. Hist., p. 71, ff. 1-3, p. 72, ff. 4-6, (1860). 

 Meek and Hayden, Pal. Upp. Mo., Smiths. Cont. Knowl., XIV, 172, p. 20, (1864) ; etc. 



Ambocoelia planoconvexa. 



Spirifer planoconvexa Shumard, Geol. Rep. Mo., p. 202, (1855); etc. 

 Ambocalia gemmula McChesney, New Pal. Foss., p. 41, (I860); ibid., 



pi. i, f. 3, (1865). 

 Spirifer \^Vartinia) }>la)io<onv( xa Meek and Hayden, Pal. Upp. Mo., 



Smiths. Cont. Knowl., xiv, 172, p. 20, ff. a-e, (1864); Meek, Fin. Rep. 



U. S. Geol. Surv. Neb., p. 184, pi. iv, f. 4, pi. vin, f. 2; etc. 

 Martinia planoconvexa McChesney, Trans. Chic. Acad. Sci., i, p. 34, 



pi. i, f. 3, 1 1868 j. 

 Ambocoelia planoconvexa Hall and Clarke, Pal. N. V., vm, pt. n, p. 5G, 



pi. xxxix, ff. 10-15, (1893). 



Meek's description : " Shell very small, plano-convex, or very 

 rarely a little concavo-convex, sometimes wider than long, in 

 other examples slightly longer than wide ; hinge margin always 

 shorter than the greatest transverse diameter of the valves, and 

 rounded at the extremities ; lateral margins and front regularly 

 rounded ; surface apparently smooth, excepting a few very ob- 

 scure concentric marks of growth, but, when examined by the 

 aid of a magnifier, showing the remains of the bases of minute 

 hair-like spines. Dorsal or smaller valve truncato-suborbicular 

 in outline, generally nearly Hat, with faint longitudinal depres- 

 sion in front, sometimes slightly convex near the beak, and con- 

 cave around the anterior and lateral margins ; beak scarcely 

 distinct from the cardinal margin ; area narrow, but well de- 

 veloped, or about half as large as in the other valve; socket 

 plates projecting like diverging teeth on each side of the small 

 fissure. Ventral valve very gibbous, particularly in the umbonal 

 region, sometimes with obscure traces of a narrow longitudinal 

 depression along the middle, but without a proper mesial sinus ; 

 beak very prominent and strongly arched back over the hinge ; 



