10 
PROCEEDINGS OE THE ACADEMY OF 
margin is regularly curved. Wings distinct, the anterior slightly inflated, 
rounded at the extremity, and separated from the body of the shell by a rather 
acute notch, from which a furrow extends to the beak ; posterior wing flat¬ 
tened, acute, subtriangular, with a shallow sinus below. Body of shell smooth ; 
wings with radiating ribs, strongest on the anterior wing and crossed by 
equally strong concentric lines ; posterior wing with fine concentric lines. 
In the oolitic limestone (“No. 3” of White.) 
Aviculopecten tenuicostus, n. sp.—Shell small, equilateral; pallial margin 
circularly rounded between anterior and posterior extremities, which lie mid¬ 
way between the beak and opposite side. Beak slightly prominent; body of 
shell bounded by a truncation from beak to each lateral margin ; anterior 
truncation slightly concave. Anterior wing of left valve moderately inflated, 
as long as anterior side of shell, distinctly rounded at extremity, joining 
hinge-line by a rounded angle, and separated from body of shell by a broadly 
Y-shaped sinus, rounded at the bottom. Posterior wing only very imperfectly 
seen. Surface (of left valve) ornamented by fine, rigid, nearly equidistant ribs, 
50 or 60 in number, separated by concave intervals ; similar but finer ribs or 
strise marking the anterior ear. Frequently from three to five equidistant cos¬ 
tate elevations appear, each of which bears two or three of the ribs. A few 
inequidistant concentric lines are seen. Right valve unknown. 
Length from beak to opposite side *47 ; antero-posterior dimension the 
same. 
It is a little singular that of seven specimens of this species all are left valves, 
showing only the anterior ear. The posterior is probably flat and thin. 
POSIDONOMYA, Brown. 
Posidonomya? ambigua, n. sp.—Shell of medium size, rather ventricose, 
somewhat oblique. Hinge-line short, straight, not surpassed by the inconspic¬ 
uous beak, abruptly rounded at the extremities ; sides of shell subparallel, 
somewhat straight; ventral margin circularly curved, gaping at the antero- 
ventral angle. Cast nearly smooth, but bearing the impression of a few small, ir¬ 
regular wrinkles around the margin. 
Greatest dimension (from beak to ventral margin) *65 (100); antero-poste¬ 
rior dimension *58 (89) ; angle of umbonal slope with hinge-line 70°. 
Three left valves and one right, of an anomalous fossil are here referred 
with great uncertainty. One of the specimens is larger and relatively longer 
from beak to venter than the one described, and seems to have been everted 
around nearly the entire pallial border, producing an extensively gaping shell. 
The right valve is a smaller specimen, with the beak near the anterior extremi¬ 
ty of the hinge-line, and presenting the anomaly of^a forward instead of a back¬ 
ward Obliquity—in this respect resembling Streblopteria , McCoy, but without 
the anterior wing. The three valves could scarcely belong to the same species 
of any genus, but it would be folly to attempt a further discrimination at 
present. 
DEXIOBIA, n. gen. 
Etymology, Jsf/df, on the right side and /?;*, strength , in allusion to the greater 
ventricosity of the right valve. 
Shell thin, inequivalve, inequilateral; beaks separated by an undefined area. 
Right valve very ventricose, with a very prominent umbo, and a produced, 
incurved beak, strongly inclined forward. Left valve much less inflated, with 
a less prominent beak, scarcely elevated above the dorsal margin. Hinge-line 
more or less extended, straight, or slightly bent, edentulous (?) furnished with a 
thickened cartilage plate bearing a linear posterior groove. Pallial line and 
muscular markings unknown. 
In his Report on the Geology of Iowa, (p. 522, pi. vii. fig. 10, a, b, c,) Prof. 
Hall has described, under the name of Cardiomorpha ovata (not C. ovata , d’Orb. 
[Jan. 
i 
