MYTILIDAE 355 



siphon and behind into a conical excurrent siphon. Foot strap-shaped. This 

 genus was formerly known as Modiolaria Beck 1838. Hinge finely dentate. 



Musculus niger Gray Black Musculus 



Plate 2 8g 



Arctic Seas to North Carolina. Alaska to Puget Sound. 



About 2 to 3 inches in length. Similar to M. discors, but much more 

 compressed and with strongly developed axial, decussated ribs on the pos- 

 terior and anterior thirds. Center section with microscopic concentric wavy 

 threads and pimples. Often pinkish on the inside. Common. 



Musculus laevigatus Gray Smooth Musculus 



Plate 28f 



Arctic Ocean to Puget Sound, and North Atlantic. 



I to I % inches in length. Distinguished from discors by its larger size, 

 and in having no pronounced radial riblet or depression separating the pos- 

 terior third from the middle area. The periostracum is more often black in 

 this species. The posterior area very often has numerous microscopic con- 

 centric scratches which, to the naked eye, give this area a dull finish. Like 

 discors, this species is much fatter than niger. 



Musculus lateralis Say Lateral Musculus 



Figure yjd 



North Carolina to Florida and the West Indies. 



% inch in length, oblong, fragile, with a center area on the valve with 

 concentric growth lines only. Remainder of shell with radial ribs. Color 

 light-brown with a strong blush of blue-green. Interior slightly iridescent. 

 Common offshore. 



Musculus discors Linne Discord Musculus 



Plate 28e 



Atlantic: Arctic Seas to Long Island Sound. Pacific: Arctic Seas to 

 Puget Sound. 



I inch in length, oblong, fairly fragile. Anterior and posterior thirds 

 of outer shell with very weak radial ribs; center section smooth except for 

 irregular growth lines. Periostracum shiny and either dark black-brown 

 or light-brown. Interior bluish white with slight iridescence. Commonly 

 dredged. 



