LAMELLIBRANCHIATA. 49] 
Ambonychia planistriata.] 
be more nearly related to Ambonychia than either of the genera mentioned. Certain 
it is that I find it more difficult to point out the distinguishing features than | did 
in those cases. The general appearance of the shells of the two genera-(Ambonychia 
and Amphicwlia) is very similar, both in the matter of form and in their surface mark- 
ings. The hinge also is very much the same in the two genera, the chief difference 
being that the area is wider in Amphicwlia. The greatest difference, however, seems 
to lie in the antero-cardinal region, where the margin of the latter is thickened, 
causing casts of the interior to appear as broadly impressed in this region. 
AMBONYCHIA PLANISTRIATA Jal. 
PLATI2 XXXV. FIGS. 3 and 4. 
Ambonychia planistriata MALL, 1861. Rep’t. Sup’t. Geol. Sur. Wis., p. 32. 
Shell obliquely acuminate-ovate or subrhomboidal, ventricose, with the point of 
greatest convexity near the center of the antero-cardinal half. Upper half of anterior 
side somewhat flattened, nearly straight, sloping backward slightly, and more rapidly 
below, into the basal margin, which, with the greater part of the posterior edge, 
forms a semicircle; postero-cardinal margin subangular, hinge line straight, one- 
third or a little less shorter than the greatest length of the shell beneath, Beaks 
prominent, strongly incurved; umbones full and rounded; posterior cardinal slope 
concave. Surface marked by distinct, broad and shallow concentric undulations 
and fine radiating striz, of which about twelve occur in 5 mm. at the margin of an 
average example. These striw, which are flattened and separated by very narrow 
interspaces, are cancellated by another set of even finer concentric lines. ‘Test very 
thin, hinge plate narrow, apparently with two cardinal teeth in each valve and no 
lateral teeth. In good casts of the interior the antero-cardinal lobe is sharply 
defined. 
This rare and beautiful species is readily distinguished from A. bellistriata Hall, 
and A. orbicularis Emmons, sp., by. its concentrically undulated surface. In this 
feature it is like Clionychia undata Emmons, sp., but that form, aside from the fact 
that it has the characters of Clionychia, is less ventricose, of somewhat different 
shape and without radiating lines. For comparisons with A. afinis Ulrich, see that 
species. 
Formation and locality.—From the ‘‘ Lower Blue limestone” at Mineral Point and Beloit, Wiscon- 
sin, and the equivalent limestones at Cannon Falls, Minnesota, and Lee county, Lilinois. 
Mus Reg. No. 8327. 
