156 TESTACEOUS MOLLUSCA. 
A. Truncata.—Byssoarca T. Sow. Z. P. 1833. Oblong, 
boat-shaped, brown, with radiating granular strize centrally and 
posteriorly, anteriorly with an obtuse keel from the umbo to the 
margin, and radiating ribs which as well as their interstices, are 
obtuse and wrinkled: posterior side very short, angulated above 
and rounded near the ventral edge, the anterior elongated and 
abruptly truncated: ligamental area elongated and broad, the 
ligament quadrangular and only attached near the posterior portion 
of the area. 11..2-3,.—Grallapagos. 
A. Avternata.—Byssoarca A. Son. in Z. P. 1833. Ob- 
long, subcylindraceous, pale brown with radiating striz and deep 
grooves at the extremities: posterior side very short, obtusely angu- 
lated dorsally, ventrally rounded; anterior, elongated, with two 
strong ribs from the umbo to the ventral margin, the anterior ribs 
strong and wrinkled; anterior marginal teeth alternating; liga- 
mental area narrow, posteriorly broader. 32. .12.—W. Colombia. 
A. Vrevata.—Byssoarca V. Sow. Z. P. 1833. Ovate, 
rather compressed, with radiating ribs, decussated; dorsal edge 
biangulated; epidermis brown, with acuminated scales: liga- 
mental area narrow. 1. .351.—Pacific. 
A. Ostiauata. Gray. in W.S. t. 2. f. 4. Ovate-subcunei- 
form, extremely short, attenuated and almost pointed posteriorly, 
rounded and expanded anteriorly, beaks close: white under a fus- 
cous scaly epidermis, with (apparently simple) close radiating rib- 
like striz, hinge margin slightly curving : an internal livid stain on 
each side. 1..12. 
A. Morasiris.—Byssoarca M. Sow. Z. P. 1833. Oblong, 
pale brown, with decussating strize : posterior side usually shorter 
sometimes nearly equal, angulated dorsally, rounded ventrally ; 
anterior side rather long, angulated dorsally, a keel from the umbo 
to the lower and anterior margin: anterior area, with larger de- 
cussated sulci. {9 .. 14%.—W. Colombia. 
A. Lirnopomus.—Byssoarca L. Sow. Z. P. 1833. Elon- 
gated, wedge-shaped, subcylindrical, with elegant decussating 
strize, posterior side the shorter and obliquely truncated ; anterior 
elongated, sloping, rounded acuminate ; ligamental area deep, 
the ligament posteriorly ending in an obtuse angle: epidermis den- 
ticulated, subfoliaceous at the posterior angle, conspicuous : teeth 
very numerous, very small and much interrupted. 1..3}. 
A. Guaciauis. Gray. in Parry. Sup. Thin, oval-elliptical, 
rounded at the sides, regularly finely and transversely striated ; 
epidermis silky : umbones towards the joint incurved and nearly 
close, the intermediate space lanceolate, ventricose ; margin even, 
thin: teeth very small and indistinct in the middle, %..3.— 
North Pole. 
