NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 75 



before the umbones ; radiating striations on the anterior part ; the lunule 

 with crenulated margin ; faint but distinctly visible radiating striae on 

 the posterior part ; and crenulations along and behind the ligament. 

 Occurrence: According to Allyn G. Smith (1944), this species is found 

 in the San Francisco Bay area. It was later collected by Dr. G. D. Hanna 

 at Coyote Point, San Francisco Bay, California, March 12, 1949, ac- 

 cording to samples in the collection of Dr. Howard H. Hill. Apparently 

 M. senhousei has been introduced into San Francisco Bay with ship- 

 ments of Japanese oysters. 

 Distribution: San Francisco Bay (introduced) ; Japan, China. 



Musculus olivaceus Dall 1916 



Plate 8, fig. 39 



Musculus olivaceus Dall, Proc. U. S. Natl. Mus., vol. 52, 1916, p. 405. 

 Holotype: U. S. National Museum. Cat. no. 210790. 

 Type loc: Off Bering Island ; 10 fms. 



Remarks: This species has ten or eleven anterior radial ribs which gradu- 

 ally become weaker; and many posterior ribs which are not bordered in 

 front by a furrow, but increase in strength backward until the ribs are 

 slightly broader than the interspaces. The fine concentric threads cross 

 the interspaces and give the median part an irregular concentric sculpture. 

 The margins are crenulated according to the sculpture; and below the 

 umbo are four strong toothlike crenulations. The ligament is short, the 

 prodissoconch is whitish and unsculptured, and the periostracum is oliva- 

 ceous with a silky lustre. Maximal length, 1 1 mm. 



Occurrence: Occurring in moderate depths, nature of bottom and if 

 nest-building unknown. 

 Distribution: Bering Sea to Catalina Island, California. 



Musculus protractus Dall 1916 



Musculus niger protractus Dall, Proc. U. S. Natl. Mus., vol. 52, 1916, 



p. 405. 

 Holotype: U. S. National Museum. Cat. no. 222017. 

 Type loc: North of Nunivak Island, Bering Sea; 9 fms. 

 Remarks: This species, which originaly was described as a variety of 

 M. niger Gray but is considered a separate species by Oldroyd (1924) 

 and Keen (1937), is more inflated and elongate than young specimens 

 of M. niger. The median area is described as being smooth and blackish. 



