Key to Family Mytilidae

Phylum Mollusca
 Class Bivalvia
  Order Mytiloida

Taken primarily from  Kozloff, 1987, 1996  p270 (Copyright 1987, 1996, University of Washington Press.  Used in this web page by permission of University of Washington Press)
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1a Shell decidedly longer than high; umbones usually at or close to the anterior end (in Adula, near the end of the first quarter) 2
1b Shell about as high or higher than long; umbones near the middle of the dorsal margin 13
2a Umbones essentially terminal; anterior ends of valves acute 3
2b Umbones not quite terminal; anterior ends of valves rounded 4
3a Valves with course radial ribs as well as irregular growth lines; periostracum of larger specimens often partly or completely worn off; length frequently exceeding 10 cm; in relatively exposed situations, mostly on the open coast  Mytilus (Mytilus) californianus
3b Valves without coarse radial ribs, and with rather regular growth lines; periostracum usually shiny black (brown in small specimens) and generally persistent; length rarely exceeding 7 cm; common in protected situations but also found on the open coast Mytilus (Mytilus) trossulus
4a Greatest height of shell usually only slightly greater than the height at the level of the umbones (except in Adula diegensis); width sometimes greater than the height; mostly burrowing in shale (except Adula diegensis, which is attached to rocks, pilings, etc.) 5
4b Greatest height of shell usually about 1.5 times the height at the level of the umbones; width not greater than the height; attached to firm substrata but not boring 8
5a Posterodorsal slopes of valves hairy (photo), and generally with an accumulation of clay particles; boring in shale or attached to rocks, pilings, etc. 6
5b Posterodorsal slopes of valves with rough, chalky encrustations and transverse wrinkles, but not hairy; boring in limestone, sometimes other rocks (not likely to be found north of California) Lithophaga (Diberus) plumula
6a Valves with filelike vertical striations; length sometimes exceeding 6 cm; boring in shale (not likely to be found north of Oregon) Adula falcata
6b Valves smooth or with fine radiating striations on their anterior portions; length not (usually) attaining 5 cm 7
7a Valves generally tapering posteriorly (the height at the level of the umbones is not usually exceeded behind the middle, but there are exceptions); genrerally boring in shale but sometimes attached to rocks, other mussels, etc. Adula californiensis
7b Valves generally higherposteriorly than near the middle (the greatest height is usually about 1.5 times the height at the level of the umbones); attached to rocks or pilings Adula diegensis
8a Length from about 2 to about 3 times the height (if slightly less than 2 times the height, then relatively large--up to more than 10 cm long) 9
8b Length about 1.5 to 1.75 times the height 11
9a Maximum length about 3 cm; periostracum greenish, often with wavy, brownish markings; periostracum not elaborated into soft bristles (introduced, along with oysters, into estuarine situations) Musculista senhousia
9b Length commonly greater than 5 cm; periostracum usually brown or blackish brown, periostracum of young speciment elaborated into soft bristles 10
10a Length about twice (or slightly less than twice) the height and width Modiolus (Modiolus) modiolus
10b Length about 3 times the height and width (not likely to be found north of California) (M. flabellatus is here considered to be a synonym) Modiolus (Modiolus) rectus

Taxonomic Levels Represented in This Key:
Adula californiensis
Modiolus (Modiolus) modiolus
Mytilus californianus
Mytilus (Mytilus) trossulus

Page created by Dave Cowles, 5-2002
Edited by Hans Helmstetler 12-2002, Dave Cowles 2005