22 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



The Californian form named M. diegensis by Coe, by some thought 

 to be M. grayanus Dunker=7kf. dunkcri Reeve introduced from Japan, 

 is a high fiat form with a very small anterior adductor and some- 

 times with a greenish shine in the periostracum. The posterior part of 

 the mantle margin has small papillae and is unpigmented in many speci- 

 mens, and thus seems to constitute a subspecies. However a large amount 

 of material was found intermediate between the lower more inflated 

 form with mantle margins more or less pigmented, and forms with dif- 

 ferent sized tentacles. Sometimes the color of the shell is brown, not 

 blue as usual. Such specimens seem to have been taken in deeper water or 

 in places where the light is weak. Where many specimens live crowded 

 together under unsuitable conditions, the specimens are small, short, and 

 often of an unusual form (71/. glomeratus Gould). 



Occurrence: M. edulis is usually found in the intertidal zone attached 

 to stones, rocks, or pilings. Sometimes specimens live in deeper water, at 

 least down to 10 to 20 fms. Fresh shells were found on the beach at San 

 Felipe, Gulf of California, but apparently they were bait brought there 

 by fishermen. 



Distribution: North America's west coast from the Arctic Ocean to 

 Cabo San Lucas, Baja California ; the west coast of South America 

 {chilensis Hupe), Valparaiso to the Strait of Magellan; the east coast 

 of South America (platensis Orbigny), north to Brazil; the east coast 

 of North America from Greenland to North Carolina (Cuba?) ; Europe 

 from the White Sea to the Mediterranean, and northern Africa; Ker- 

 guelen Island (desolationis Lamy) ; Australia and New Zealand (planu- 

 latus Lamarck). 



Mytilus calif ornianus Conrad 1837 

 Plate 1, figs. 3-4; text-figs. 3, 12 



Mytilus calif ornianus Conrad, Jour. Acad. Nat. Sci. Phila., vol. 7, 1837, 



p. 242, PI. 18, fig. 15. 

 Syn.: Mytilus calif ornicus Clessin 1889. 

 Holotype: ? 



Type loc: San Diego, California. 



Remarks: This species is usually easy to recognize by the radiating ribs. 

 In shape, however, it varies from the broad bay form to the elongate ir- 

 regular and worn form living in the surf. Small specimens may some- 

 times be difficult to separate from small specimens of Mytilus edulis, but 

 the placement of the anterior adductor is different. In M. calif ornianus. 



