156 TESTACEOUS MOLLUSCA. 



A. Truncata. — Byssoarca T. Sow. Z. P. 1833. Oblong, 

 boat-shaped, brown, with radiating granular striae 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. 1^-. . 2-^. — Gallapagos. 



A. Alternata. — Byssoarca A. Sow. in Z. P. 1833. Ob- 

 long, subcylindraceous, pale brown with radiating striae 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. J^ . . 1-f. — W. Colombia. 



A. Velata. — 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 . . 3 T ^. — Pacific. 



A. Obliquata. 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 striae, hinge margin slightly curving : an internal livid stain on 

 each side. 1. . 1^. 



A. Mutabilis.— Byssoarca M. Sow. Z. P. 1833. Oblong, 

 pale brown, with decussating striae : 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. $j . . I-^q.—W. Colombia. 



A. Lithodomus. — Byssoarca L. Sow. Z. P. 1833. Elon- 

 gated, wedge-shaped, subcylindrical, with elegant decussating 

 striae, 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. Glacialis. 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, f . . £ , — 

 North Pole. 



