LAMELLIBRANCHIATA. 493 
Ambonychia affinis.] 
more prominent, giving it a more erect appearance than any of the other species 
referred to the genus. Nor can I find that A. orbicularis ever has concentric undu- 
ations. 
Formation and locality—Middle Galena, Weisbach’s dam, near Spring Valley, Minnesota; also in 
Carroll county, Llinois. 
Mus. Reg. No. 83438. 
AMBONYCHIA AMYGDALINA J/all. 
PLATE XXXV, FIGS. 8 and 9. 
Ambonychia amygdalina HAL, 1847. Pal. N. Y., vol. i, p. 165. 
None of the specimens seen by me preserve the surface characters well enough 
to prove that this species was provided with radiating lines. Obscure traces of such 
strive are to be made out on one of the casts of the interior, but the evidence is not 
sufficient for me to assert that they are what they seem. Still, it is highly probable 
that radiating striz will be found on perfect specimens, in which case the species 
would stand very near A. bellistriata, differing from it, so far as we can now see, 
chiefly in its greater size, less incurved beaks, flatter anterior side and less angular 
postero-cardinal margin. 
The anterior lobe is longer, more sunken and less sharply defined in this species 
than in the others here referred to Ambonychia. 
Formation and locality.—Middle Galena of Goodhue county, Minnesota. The New York type of 
the species is credited to the Trenton limestone at Adams, Jefferson county. Billings also catalogues 
the species from the same horizon in Canada. 
Genus CLIONYCHIA, Ulrich. 
Ambonychia (part.) HALL, 1847. Pal. N. Y., vol. i, p. 163. 
Clionychia, Ubnicu, 1892. American Geologist, vol. x, p. 97; Clionychia, MILLER, 1892. First Ap- 
pendix, N. A. Geol. and Pal., p. 699. 
Shells equivalve, moderately covvex, subalate posteriorly; beaks terminal, com- 
paratively small, not very prominent and but little incurved. Cardinal line straight, 
rather long, forming an angle of less than 90° with the anterior side. Surface marked 
concentrically only. No byssal opening, the margins closing tightly all around. 
Muscular impressions situated in the postero-cardinal third, large, bilobed, the 
lower lobe much larger than the upper. Pallial line simple, extending from the 
posterior adductor to the rostral cavity. Hinge plate of moderate strength, without 
cardinal or lateral teeth, excavated longitudinally for a linear ligament. Upper 
part of anterior edge thickened, producing a more or less well-marked impression 
in this part of casts of the interior. Anterior pedal muscle attached a short distance 
behind the beaks. 
Type: Ambonychia lamellosa Hall. 
