70 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



The largest specimen (1012-39) measures 21,5 mm in length, 11.9 

 mm in height, and 6.1 mm in diameter. 



This species is very close to Modiolus (Amygdalum) sagittatus Rehder 

 1935 ; from off Florida, the only differences seem to be the narrower 

 anterior part and the more convex ventral margin. Several specimens had 

 small commensal crabs (Pinnotheres) in the mantle cavity. 

 Occurrence: A. paUidulum has a wide bathymetric range, as it is recorded 

 taken alive by shore collecting and down to 210 fms. Usually the bottom 

 consists of mud or fine sand, where the species lives in a "nest" made of 

 fine byssal threads and mud particles. The Hancock material extends the 

 distribution from Acapulco, Mexico, south to off the coast of Colombia. 

 Distribution: Off Bodega Head, California, to off the coast of Colombia, 

 SWof Isla Gorgona (1°02'30''N, 81°12'W). 



Amygdalum americanum new species 

 Plate 8, fig. 37 



Diagnosis: The shell is thin but not translucent, elongate, with the um- 

 bones close to the anterior end. The posterior part is broader than the 

 anterior, but the dorsal and ventral margins form only a small angle 

 when continued forward. The prodissoconch is large, colorless or whitish, 

 shining and distinctly set off from the rest of the shell. The anterior mar- 

 gin is rounded and slopes evenly to the ventral margin ; the posterior 

 margin is truncate, with rounded dorsal and ventral angles. The sculp- 

 ture consists of fine concentric growth lines, more irregular ventrally; 

 the periostracum is yellowish, the anterior and ventral parts being of a 

 uniform yellowish-white color; in the upper part, fine hyaline lines run 

 from the umbo backward, where they become obsolete; and some very 

 fine brownish lines cross the growth lines toward the ventral margin in 

 the anterior part. Grayish-brown triangular spots give a speckled appear- 

 ance to the posterior and hinder dorsal parts of the shell. Some spots form 

 irregular blotches along the dorsal margin, or bandlike figures toward the 

 ventral margin. These rather variable color spots are not seen in small 

 specimens, but in them the hyaline lines are very distinct. The interior 

 is white and margaritaceous, with fine radiating striations; the muscle 

 scars are indistinct, but the muscles are placed the same way as in A. pal- 

 lidulum. The posterior adductor is slightly larger, and the anterior ad- 

 ductor is closer to the umbones. The foot is placed in the anterior third 

 of the shell, the byssal gland small. The mantle margins are thickened in 

 the posterior branchial part and they and the septum are speckled with 



