CLASS PELECYPODA 
121 
Range. Bering Strait to San Diego, California, also Atlantic. In 
the Pleistocene and Pliocene at Santa Barbara and the Pliocene at San 
Diego Well, and San Pedro, California. 
Thyasira trisinuata Orbigny, 1846. 
Moll. Cuba, 2:300; pi. 27, figs. 26, 49. 
Testa transversa, inflata, laevigata, albida, tenui, diaphana; latere 
buccali brevi, truncato; latere anali trisinuato, radiatim bicostato. Diame¬ 
ter, 4 mm. (Orbigny.) 
Type in British Museum. Type locality, Martinique and Guadeloupe. 
Range. Sitka, Alaska, to San Diego, California. Also Atlantic. 
Thyasira trisinuata polygona Jeffreys, 1863. 
British Conch., 2:248. 
Rather longer in proportion to the breadth, marked by three or four 
longitudinal ridges, giving the shell an angular form, the grooves on the 
posterior side being much deeper than usual. (Jeffreys.) 
Type locality not given. 
Range. Korea and the Gulf of Alaska to San Diego, California. 
Also Atlantic. 
Thyasira cygnus Dali, 1916. 
Plate 3, fig. 10. 
Proc. U. S. N. M., 52:409. 
Shell white with a pale straw-colored periostracum, moderately con¬ 
vex, sharply compressed behind, the beaks prominent, prosoccelous over 
a large cordate lunule, the escutcheon long and very narrow, bounded by 
a sharp keel; general form rounded quadrate, the compressed posterior 
area narrow and basally falling notably short of the basal curve of the 
disk; posterior slope slightly convexly arcuate; anterior distinctly concave, 
meeting the basal arc in an obtuse angulation. Length, 14; height, 13; 
diameter 8.5 mm. (Dali.) 
Type in U. S. N. M., No. 222618. Type locality, Station 4224, Cygnet 
Inlet, Boca de Quadra, Alaska, in 160 fathoms. 
Range. Southeastern Alaska. 
Thyasira excavata Dali, 1901. 
Proc. U. S. N. M., 23:818; pi. 39, figs. 12, 15. 
Shell subovate, thin, white, with a pale yellowish periostracum; sculp¬ 
ture of concentric incremental lines, and in each valve three sharp and 
two or three obscure radial ridges. Beaks small, sub arcuate, not 
prominent, distinctly prosogyrate; lunule and escutcheon well developed, 
elongate, rather narrow, and emphatically excavated, bounded by a well- 
